Mammal
Encyclopedia
Mammals are members of a class
of air-breathing vertebrate
animal
s characterised by the possession of endothermy, hair
, three middle ear
bone
s, and mammary glands functional in mothers with young. Most mammals also possess sweat glands and specialised teeth, and the largest group of mammals, the placentals
, have a placenta
which feeds the offspring during gestation. The mammalian brain, with its characteristic neocortex
, regulates endothermic
and circulatory
systems, the latter featuring red blood cells lacking nuclei and a large four-chambered heart
maintaining the very high metabolism rate they have. Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) Bumblebee Bat
to the 33-meter (108-foot) Blue Whale
.
The word "mammal" comes from the Latin
word "mamma" meaning "breast". All female mammals nurse their young with milk which comes out from special glands called mammary glands.
According to Mammal Species of the World, which is updated through periodic editions, 5676 species
were known in 2005, distributed in 1,229 genera
, 153 families
and 29 orders
. In 2008 the IUCN completed a 5-year, 17,000 scientist Global Mammal Assessment for its IUCN Red List
, which counted 5488 accepted species at the end of that period. The class is divided into two subclasses (not counting fossils): the Prototheria
(order of Monotremata) and the Theria
, the latter containing the infraclasses Metatheria
(including marsupials) and Eutheria
(the placentals). The classification of mammals
between the relatively stable class and family levels having changed often, different treatments of subclass, infraclass and order appear in contemporaneous literature, especially for Marsupialia.
Except for the five species of monotreme
s (which lay eggs), all living mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders, in descending order, are Rodentia (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), Chiroptera (bats), and Soricomorpha
(shrews, moles and solenodons). The next three largest orders, depending on classification scheme
, include the Primates, to which the human
species belongs, the Cetartiodactyla
(including the even-toed hoofed mammals and the whales) and the Carnivora
(dogs, cats, weasels, bears, seals, and their relatives).
The early synapsid
mammalian ancestors, a group which included pelycosaur
s such as Dimetrodon
, diverged from the amniote
line that would lead to reptile
s at the end of the Carboniferous
period. Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first true mammals appeared 220 million years ago in the Triassic
period. Modern mammalian orders appeared in the Palaeocene and Eocene
epochs of the Palaeogene period. Phylogenetically
, the clade
Mammalia is defined as all descendants of the most recent common ancestor
of monotremes (e.g., echidna
s and platypus
es) and therian mammals (marsupial
s and placentals). This means that some extinct groups of "mammals" are not members of the crown group
Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. These "mammals" are now usually placed in the unranked clade Mammaliaformes
.
to produce
milk
. However, other features are required when classifying fossil
s, since soft tissue glands and some other features are not visible in fossils. Paleontologists
use a distinguishing feature that is shared by all living mammals (including monotremes), but is not present in any of the early Triassic
synapsids: mammals use two bones for hearing that were used for eating by their ancestors. The earliest synapsids had a jaw joint composed of the articular
(a small bone at the back of the lower jaw) and the quadrate
(a small bone at the back of the upper jaw). Most reptiles including lizards, crocodilians, dinosaur
s use this system, as did non-mammalian synapsids such as therapsids. Mammals have a different jaw joint, however, composed only of the dentary (the lower jaw bone which carries the teeth) and the squamosal
(another small skull bone). In mammals the quadrate and articular bones have become the incus
and malleus
bones in the middle ear
.
Mammals also have a double occipital condyle
: they have two knobs at the base of the skull which fit into the topmost neck vertebra, and other vertebrates have a single occipital condyle. Paleontologists use only the jaw joint and middle ear as criteria for identifying fossil mammals, since it would be confusing if they found a fossil that had one feature, but not the other.
George Gaylord Simpson
's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) was the original source for the taxonomy listed here. Simpson laid out a systematics of mammal origins and relationships that was universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics
. Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals.
classroom textbooks. The following taxonomy of extant and recently extinct mammals is from Vaughan et al. (2000).
Class Mammalia
and Susan K. Bell, which has resulted in the "McKenna/Bell classification". Their 1997 book, Classification of Mammals: Above the species level, is the most comprehensive work to date on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus. The new McKenna/Bell classification was quickly accepted by paleontologists, though recent molecular genetic data challenge several of the higher level groupings. The authors work together as paleontologists at the American Museum of Natural History
, New York
. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.
The McKenna/Bell hierarchical listing of all of the terms used for mammal groups above the species includes extinct mammals as well as modern groups, and introduces some fine distinctions such as legions
and sublegions (ranks which fall between classes and orders) that are likely to be glossed over by the nonprofessionals.
The published re-classification forms both a comprehensive and authoritative record of approved names and classifications and a list of invalid names.
Extinct groups are represented by a dagger
(†).
Class Mammalia
analysis have suggested new relationships among mammal families over the last few years. Most of these findings have been independently validated by retrotransposon
presence/absence data
. The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals. Molecular clock
s suggest that these clades diverged from early common ancestors in the Cretaceous
, but fossils have not yet been found to corroborate this hypothesis. These molecular findings are consistent with mammal zoogeography
:
Following molecular DNA sequence analyses, the first divergence was that of the Afrotheria
110–100 million years ago. The Afrotheria proceeded to evolve and diversify in the isolation of the African-Arabian continent. The Xenarthra
, isolated in South America
, diverged from the Boreoeutheria
approximately 100–95 million years ago. According to an alternative view, the Xenarthra has the Afrotheria as closest allies, forming the Atlantogenata
as sistergroup to Boreoeutheria. The Boreoeutheria split into the Laurasiatheria
and Euarchontoglires
between 95 and 85 mya; both of these groups evolved on the northern continent of Laurasia
. After tens of millions of years of relative isolation, Africa-Arabia collided with Eurasia, exchanging Afrotheria and Boreoeutheria. The formation of the Isthmus of Panama
linked South America
and North America
, which facilitated the exchange of mammal species in the Great American Interchange
. The traditional view that no placental mammals reached Australasia
until about 5 million years ago when bats and murine
rodents arrived has been challenged by recent evidence and may need to be reassessed. These molecular results are still controversial because they are not reflected by morphological
data, and thus not accepted by many systematists. Further there is some indication from retrotransposon presence/absence data that the traditional Epitheria
hypothesis, suggesting Xenarthra
as the first divergence, might be true. With the old order Insectivora
shown to be polyphylectic
and more properly subdivided (as Afrosoricida
, Erinaceomorpha
, and Soricomorpha
), the following classification for placental mammals contains 21 orders:
Synapsida, the group which contains mammals and their extinct relatives, originated during the Pennsylvanian
epoch
, when they split from the lineage that led to reptiles and birds. Non-mammalian synapsids were once called "mammal-like reptiles", although they are usually no longer considered reptiles. Mammals evolved from non-mammalian synapsids during the Early Jurassic
.
The first fully terrestrial vertebrate
s were amniote
s. Like their amphibian
predecessors, they have lungs and limbs. Amniotes' eggs, however, have internal membranes which allow the developing embryo
to breathe but keep water in. Hence amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water.
The first amniotes apparently arose in the late
Carboniferous
. They descended from earlier reptiliomorph
amphibians, which lived on land already inhabited by insect
s and other invertebrates, and by fern
s, moss
es and other plants. Within a few million years two important amniote lineages became distinct: the synapsid
s, which include mammals; and the sauropsids, which include lizard
s, snake
s, crocodilians, dinosaur
s and bird
s. Synapsids have a single hole (temporal fenestra) low on each side of the skull.
One synapsid group, the pelycosaur
s, were the most common land vertebrates of the early Permian
and included the largest land animals of the time.
Therapsids descended from pelycosaurs in the middle Permian
, about 260M years ago, and took over their position as the dominant land vertebrates. They differ from pelycosaurs in several features of the skull and jaws, including: larger temporal fenestrae and incisor
s which are equal in size. The therapsids went through a series of stages, beginning with animals which were very like their pelycosaur ancestors and ending with the Triassic cynodont
s, some of which could easily be mistaken for mammals. Those stages were characterized by:
s, dinosaur
s and bird
s. After this "Triassic Takeover" the cynodonts and their descendants
could only survive as small, mainly nocturnal insectivore
s.
The first true mammals appeared in the Late Triassic (ca. 200 million years ago), over 70 million years after the first therapsids and approximately 30 million years after the first mammaliaformes. The rat-like Hadrocodium
appears to be in the middle of the transition to true mammal status — it had a mammalian jaw joint (formed by the dentary and squamosal bones), but there is some debate about whether its middle ear
was fully mammalian. The majority of the mammal species that existed in the Mesozoic Era
were characterized by Multituberculates.
The earliest known monotreme
is Teinolophos
, which lived about 123M years ago in Australia
. Monotremes have some features which may be inherited from the original amniote
s:
Unlike other mammals, female monotremes do not have nipples and feed their young by "sweating" milk from patches on their bellies.
The oldest known marsupial
is Sinodelphys
, found in 125M-year old early Cretaceous
shale
in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.
The living Eutheria
("true beasts") are all placentals. An early eutherian, Eomaia
, found in China and dated to 125M years ago, obtained some features which are more like those of marsupials, which suggested it was perhaps a transitional fossil that eventually gave rise to the placental lineage (the surviving metatherians):
Now the discovery of a partial skeleton of a small, shrewlike mammal, described online in Nature in August of 2011, pushes back the date of the divergence by 30 million years, to 160 million years ago. Found in the famous fossil beds of Liaoning, China, the newly discovered little mammal has been named Juramaia sinensis, or "Jurassic mother from China."
It is not certain when true placental mammals evolved – the earliest undisputed fossils of placentals come from the early Paleocene
, after the extinction of the dinosaurs.
Mammals and near-mammals expanded out of their nocturnal insectivore niche from the mid Jurassic onwards – for example Castorocauda had adaptations for swimming, digging and catching fish.
, after the Cretaceous–Tertiary extinction event emptied ecological space once filled by reptiles. Then mammals diversified very quickly, both birds and mammals show an exponential rise in diversity. An example is that the earliest known bat dates from about 50M years ago, only 15M years after the extinction of the dinosaurs.
Recent molecular phylogenetic studies suggest that most placental orders
diverged about 100M to 85M years ago, but that modern families
first appeared in the late Eocene
and early Miocene
But paleontologists object that no placental fossils have been found from before the end of the Cretaceous.
During the Cenozoic
several groups of mammals appeared which were much larger than their nearest modern equivalents – but none was even close to the size of the largest dinosaurs with similar feeding habits.
, whose fossils date from the early Jurassic
(approx. 195 million years ago), provides the first clear evidence of fully mammalian jaw joints.
It has been suggested that the original function of lactation (milk production) was to keep eggs moist. Much of the argument is based on monotremes (egg-laying mammals).
The earliest clear evidence of hair or fur is in fossils of Castorocauda, from 164M years ago in the mid Jurassic
. From 1955 onwards some scientists have interpreted the foramina (passages) in the maxilla
e (upper jaws) and premaxilla
e (small bones in front of the maxillae) of cynodont
s as channels which supplied blood vessels and nerves to vibrissae (whiskers), and suggested that this was evidence of hair or fur. But foramina do not necessarily show that an animal had vibrissae – for example the modern lizard Tupinambis has foramina which are almost identical to those found in the non-mammalian cynodont Thrinaxodon
.
The evolution of erect limbs in mammals is incomplete — living and fossil monotremes have sprawling limbs. In fact some scientists think that the parasagittal (non-sprawling) limb posture is a synapomorphy
(distinguishing characteristic) of the Boreosphenida
, a group which contains the Theria
and therefore includes the last common ancestor of modern marsupial and placentals – and therefore that all earlier mammals had sprawling limbs. Sinodelphys
(the earliest known marsupial) and Eomaia
(the earliest known eutheria
n) lived about 125M years ago, so erect limbs must have evolved before then.
It is currently very difficult to be confident when endothermy first appeared in the evolution of mammals. Modern monotremes have a lower body temperature and more variable metabolic rate than marsupials and placentals. So the main question is when a monotreme-like metabolism evolved in mammals. The evidence found so far suggests Triassic
cynodont
s may have had fairly high metabolic rates, but is not conclusive. In particular it is difficult to see how small animals can maintain a high and stable body temperature without fur.
(bones in the neck
); this includes bat
s, giraffe
s, whale
s, and human
s. The few exceptions include the manatee
and the two-toed sloth
, which have only six cervical vertebrae, and the three-toed sloth
with nine cervical vertebrae.
having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans
are typical of this type of lung.
Breathing is largely driven by the muscular diaphragm which divides the thorax from the abdominal cavity, forming a dome with its convexity towards the thorax. Contraction of the diaphragm flattens the dome increasing the volume of the cavity in which the lung is enclosed. Air enters through the oral and nasal cavities; it flows through the larynx, trachea and bronchi and expands the alveoli. Relaxation of the diaphragm has the opposite effect, passively recoiling during normal breathing. During exercise, the abdominal wall contracts
, increasing visceral pressure on the diaphragm, thus forcing the air out more quickly and forcefully. The rib cage
itself also is able to expand and contract the thoracic cavity to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its pressure gradient. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows
. Mammals take oxygen into their lungs, and discard carbon dioxide.
, a brain region that is unique to mammals. Placental mammals have a corpus callosum
unlike monotremes and marsupials. The size and number of cortical areas (Brodmann
's areas) is least in monotremes (about 8-10) and most in placentals (up to 50).
is made up of three layers: the outermost epidermis, the dermis
, and the hypodermis
.
The epidermis is typically ten to thirty cells thick, its main function being to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is fifteen to forty times thicker than the epidermis. The dermis is made up of many components such as bony structures and blood vessels. The hypodermis is made up of adipose tissue
. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species.
Although mammals and other animals have cilia that superficially may resemble it, no other animals except mammals have hair
. It is a definitive characteristic of the class. Some mammals have very little, but nonetheless, careful examination reveals the characteristic, often in obscure parts of their bodies. None are known to have hair that naturally is blue or green in color although some cetaceans, along with the mandrill
s, appear to have shades of blue skin. Many mammals are indicated as having blue hair or fur, but in all known cases, it has been found to be a shade of gray. The two-toed sloth
and the polar bear
may seem to have green fur, but this color is caused by algae
growths.
Most mammals give birth to live young (vivipary
), but a few, namely the monotremes, lay eggs
. The platypus
and the echidna
present a particular sex determination system
that is different from other vertebrates.
Certain glands of mammals known as mammary gland
s are specialized to produce milk
, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the nipple
s seen in most mammals, but they do have mammary glands.
Viviparous mammals are classified into the subclass Theria and are divided into two infraclasses: Metatheria
(of which only the Marsupialia survive), and Eutheria. Marsupialia, or marsupial
s, have short gestation
periods and give birth to undeveloped young which are contained within a pouch-like sac, (marsupium
), located in the front of the mothers' abdomen
. Eutherians, commonly known as placentals, are mammals that give birth to complete and fully developed young. This is usually characterized by long gestation
periods. The majority of mammal species are classified as eutherians.
"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in cold weather and climates where non-avian reptiles and large insects cannot.
Endothermy requires plenty of food energy, so pound for pound mammals eat more food than most reptiles. Small insectivorous mammals eat prodigious amounts for their size.
A rare exception, the naked mole rat
produces little metabolic heat, so it is considered an operational poikilotherm
. Bird
s are also endothermic, so endothermy is not a defining mammalian feature.
In intelligent mammals, such as primates, the cerebrum is larger relative to the rest of the brain. Intelligence
itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rats
, for example, are considered to be highly intelligent as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat
. In some mammals, food gathering appears to be related to intelligence: a deer
feeding on plant
s has a brain
smaller than a cat
, which must think to outwit its prey.
s travel slowly along branches rather than swinging energetically like monkeys.
have since adapted to meet their dietary requirements in a variety of ways. Some eat other animals – this is a carnivorous
diet (and includes insectivorous
diets). Other mammals, called herbivores, eat plants. A herbivorous diet includes sub-types such as fruit-eating and grass-eating. An omnivore
eats both prey and plants. Carnivorous mammals have a simple digestive tract, because the proteins, lipids, and minerals found in meat require little in the way of specialized digestion. Plants, on the other hand, contain complex carbohydrates, such as cellulose
. The digestive tract of an herbivore is therefore host to bacteria
that ferment these substances, and make them available for digestion. The bacteria are either housed in the multi-chambered stomach
or in a large cecum
. The size of an animal is also a factor in determining diet type. Since small mammals have a high ratio of heat-losing surface area to heat-generating volume, they tend to have high-energy requirements and a high metabolic rate
. Mammals that weigh less than about 18 oz (500 g) are mostly insectivorous because they cannot tolerate the slow, complex digestive process of a herbivore. Larger animals on the other hand generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (those that prey on larger vertebrates) or a slower digestive process (herbivores). Furthermore, mammals that weigh more than 18 oz (500 g) usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).
Specializations in herbivory include:
Granivory "seed eating", folivory "leaf eating", fruivory "fruit eating", nectivory "nectar eating", gumivory "gum eating", and mycophagy "fungus eating".
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of air-breathing vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s characterised by the possession of endothermy, hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
, three middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
s, and mammary glands functional in mothers with young. Most mammals also possess sweat glands and specialised teeth, and the largest group of mammals, the placentals
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
, have a placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
which feeds the offspring during gestation. The mammalian brain, with its characteristic neocortex
Neocortex
The neocortex , also called the neopallium and isocortex , is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI...
, regulates endothermic
Warm-blooded
The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...
and circulatory
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
systems, the latter featuring red blood cells lacking nuclei and a large four-chambered heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
maintaining the very high metabolism rate they have. Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) Bumblebee Bat
Bumblebee Bat
Kitti's Hog-nosed Bat , also known as the bumblebee bat, is a vulnerable species of bat and the only extant member of the family Craseonycteridae. It occurs in western Thailand and southeast Burma, where it occupies limestone caves along rivers.Kitti's Hog-nosed Bat is the smallest species of bat...
to the 33-meter (108-foot) Blue Whale
Blue Whale
The blue whale is a marine mammal belonging to the suborder of baleen whales . At in length and or more in weight, it is the largest known animal to have ever existed....
.
The word "mammal" comes from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
word "mamma" meaning "breast". All female mammals nurse their young with milk which comes out from special glands called mammary glands.
According to Mammal Species of the World, which is updated through periodic editions, 5676 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
were known in 2005, distributed in 1,229 genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
, 153 families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
and 29 orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
. In 2008 the IUCN completed a 5-year, 17,000 scientist Global Mammal Assessment for its IUCN Red List
IUCN Red List
The IUCN Red List of Threatened Species , founded in 1963, is the world's most comprehensive inventory of the global conservation status of biological species. The International Union for Conservation of Nature is the world's main authority on the conservation status of species...
, which counted 5488 accepted species at the end of that period. The class is divided into two subclasses (not counting fossils): the Prototheria
Prototheria
Prototheria is a taxonomic group, or taxon, to which the order Monotremata belongs. It is conventionally ranked as a subclass within the mammals.Most of the animals in this group are extinct...
(order of Monotremata) and the Theria
Theria
Theria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....
, the latter containing the infraclasses Metatheria
Metatheria
Metatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupialia though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals.The earliest known...
(including marsupials) and Eutheria
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
(the placentals). The classification of mammals
Mammal classification
Mammalia is a class of animal within the Phylum Chordata. Mammal classification has been through several iterations since Carolus Linnaeus initially defined the class. Many earlier ideas have been completely abandoned by modern taxonomists, among these are the idea that bats are related to birds...
between the relatively stable class and family levels having changed often, different treatments of subclass, infraclass and order appear in contemporaneous literature, especially for Marsupialia.
Except for the five species of monotreme
Monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s (which lay eggs), all living mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders, in descending order, are Rodentia (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), Chiroptera (bats), and Soricomorpha
Soricomorpha
The order Soricomorpha is taxon within the class of mammals. In previous years it formed a significant group within the former order Insectivora...
(shrews, moles and solenodons). The next three largest orders, depending on classification scheme
Biological classification
Biological classification, or scientific classification in biology, is a method to group and categorize organisms by biological type, such as genus or species. Biological classification is part of scientific taxonomy....
, include the Primates, to which the human
Human
Humans are the only living species in the Homo genus...
species belongs, the Cetartiodactyla
Cetartiodactyla
Cetartiodactyla is the clade in which whales and even-toed ungulates have currently been placed. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. The term Cetartiodactyla reflects the idea that whales evolved within the artiodactyls...
(including the even-toed hoofed mammals and the whales) and the Carnivora
Carnivora
The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...
(dogs, cats, weasels, bears, seals, and their relatives).
The early synapsid
Synapsid
Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...
mammalian ancestors, a group which included pelycosaur
Pelycosaur
The pelycosaurs are an informal grouping composed of basal or primitive Late Paleozoic synapsid amniotes. Some species were quite large and could grow up to 3 meters or more, although most species were much smaller...
s such as Dimetrodon
Dimetrodon
Dimetrodon was a predatory synapsid genus that flourished during the Permian period, living between 280–265 million years ago ....
, diverged from the amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
line that would lead to reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s at the end of the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
period. Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first true mammals appeared 220 million years ago in the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
period. Modern mammalian orders appeared in the Palaeocene and Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
epochs of the Palaeogene period. Phylogenetically
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
, the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
Mammalia is defined as all descendants of the most recent common ancestor
Most recent common ancestor
In genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...
of monotremes (e.g., echidna
Echidna
Echidnas , also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs...
s and platypus
Platypus
The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
es) and therian mammals (marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s and placentals). This means that some extinct groups of "mammals" are not members of the crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. These "mammals" are now usually placed in the unranked clade Mammaliaformes
Mammaliaformes
Mammaliaformes is a clade that contains the mammals and their closest extinct relatives. Phylogenetically, it is defined as a clade including the most recent common ancestor of Sinoconodon, morganuconodonts, docodonts, Monotremata, Marsupialia, Placentalia, extinct members of this clade, and all...
.
Distinguishing features
Living mammal species can be identified by the presence of sweat glands, including those that are specializedNipple
In its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts or udder of a mammal by which breast milk is delivered to a mother's young. In this sense, it is often called a teat, especially when referring to non-humans, and...
to produce
Lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...
milk
Milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
. However, other features are required when classifying fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s, since soft tissue glands and some other features are not visible in fossils. Paleontologists
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
use a distinguishing feature that is shared by all living mammals (including monotremes), but is not present in any of the early Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
synapsids: mammals use two bones for hearing that were used for eating by their ancestors. The earliest synapsids had a jaw joint composed of the articular
Articular
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...
(a small bone at the back of the lower jaw) and the quadrate
Quadrate bone
The quadrate bone is part of a skull in most tetrapods, including amphibians, sauropsids , and early synapsids. In these animals it connects to the quadratojugal and squamosal in the skull, and forms part of the jaw joint .- Evolutionary variation :In snakes, the quadrate bone has become elongated...
(a small bone at the back of the upper jaw). Most reptiles including lizards, crocodilians, dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s use this system, as did non-mammalian synapsids such as therapsids. Mammals have a different jaw joint, however, composed only of the dentary (the lower jaw bone which carries the teeth) and the squamosal
Squamosal
The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...
(another small skull bone). In mammals the quadrate and articular bones have become the incus
Incus
The incus or anvil is the anvil-shaped small bone or ossicle in themiddle ear. It connects the malleus to the stapes. It was first described by Alessandro Achillini of Bologna.The incus transmits sound vibrations from the malleus to the stapes....
and malleus
Malleus
The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
bones in the middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
.
Mammals also have a double occipital condyle
Occipital condyle
The occipital condyles are undersurface facets of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra....
: they have two knobs at the base of the skull which fit into the topmost neck vertebra, and other vertebrates have a single occipital condyle. Paleontologists use only the jaw joint and middle ear as criteria for identifying fossil mammals, since it would be confusing if they found a fossil that had one feature, but not the other.
Classification
George Gaylord Simpson
George Gaylord Simpson was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern evolutionary synthesis, contributing Tempo and mode in evolution , The meaning of evolution and The major features of...
's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) was the original source for the taxonomy listed here. Simpson laid out a systematics of mammal origins and relationships that was universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
. Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals.
Standardized textbook classification
A somewhat standardized classification system has been adopted by most current mammalogyMammalogy
In zoology, mammalogy is the study of mammals – a class of vertebrates with characteristics such as homeothermic metabolism, fur, four-chambered hearts, and complex nervous systems...
classroom textbooks. The following taxonomy of extant and recently extinct mammals is from Vaughan et al. (2000).
Class Mammalia
- Subclass PrototheriaPrototheriaPrototheria is a taxonomic group, or taxon, to which the order Monotremata belongs. It is conventionally ranked as a subclass within the mammals.Most of the animals in this group are extinct...
: monotremes: platypusPlatypusThe platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
es and echidnaEchidnaEchidnas , also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs...
s. - Subclass TheriaTheriaTheria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....
: live-bearing mammals- Infraclass MetatheriaMetatheriaMetatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupialia though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals.The earliest known...
: marsupials - Infraclass EutheriaEutheriaEutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
: placentals
- Infraclass Metatheria
McKenna/Bell classification
In 1997, the mammals were comprehensively revised by Malcolm C. McKennaMalcolm McKenna
Malcolm Carnegie McKenna was an American paleontologist. He was the curator of vertebrate paleontology at the American Museum of Natural History and co-authored the book Classification of Mammals along with Susan K. Bell...
and Susan K. Bell, which has resulted in the "McKenna/Bell classification". Their 1997 book, Classification of Mammals: Above the species level, is the most comprehensive work to date on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus. The new McKenna/Bell classification was quickly accepted by paleontologists, though recent molecular genetic data challenge several of the higher level groupings. The authors work together as paleontologists at the American Museum of Natural History
American Museum of Natural History
The American Museum of Natural History , located on the Upper West Side of Manhattan in New York City, United States, is one of the largest and most celebrated museums in the world...
, New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.
The McKenna/Bell hierarchical listing of all of the terms used for mammal groups above the species includes extinct mammals as well as modern groups, and introduces some fine distinctions such as legions
Legion (biology)
The legion, in biological taxonomy, is a non-obligatory taxonomic rank within the Linnaean hierarchy which is subordinate to the class but superordinate to the cohort...
and sublegions (ranks which fall between classes and orders) that are likely to be glossed over by the nonprofessionals.
The published re-classification forms both a comprehensive and authoritative record of approved names and classifications and a list of invalid names.
Extinct groups are represented by a dagger
Dagger (typography)
A dagger, or obelisk. is a typographical symbol or glyph. The term "obelisk" derives from Greek , which means "little obelus"; from meaning "roasting spit"...
(†).
Class Mammalia
- Subclass PrototheriaPrototheriaPrototheria is a taxonomic group, or taxon, to which the order Monotremata belongs. It is conventionally ranked as a subclass within the mammals.Most of the animals in this group are extinct...
: monotremes: echidnaEchidnaEchidnas , also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs...
s and the PlatypusPlatypusThe platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young... - Subclass Theriiformes: live-bearing mammals and their prehistoric relatives
- Infraclass †AllotheriaAllotheriaAllotheria was a branch of successful Mesozoic mammals. The most important characteristic was the presence of lower molariform teeth equipped with two longitudinal rows of cusps...
: multituberculates - Infraclass †TriconodontaTriconodontaTriconodonta is the generic name for a group of early mammals which were close relatives of the ancestors of all present-day mammals. Triconodonts lived between the Triassic and the Cretaceous. They are one of the groups that can be classified as mammals by any definition...
: triconodonts - Infraclass Holotheria: modern live-bearing mammals and their prehistoric relatives
- Supercohort TheriaTheriaTheria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....
: live-bearing mammals- Cohort Marsupialia: marsupials
- Magnorder AustralidelphiaAustralidelphiaAustralidelphia is the superorder that contains roughly three-quarters of all marsupials, including all those native to Australasia and a single species from South America...
: AustraliaAustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
n marsupials and the Monito del MonteMonito del MonteThe Monito del Monte The Monito del Monte The Monito del Monte (Spanish for "little mountain monkey", Dromiciops gliroides, is a diminutive marsupial native only to southwestern South America (Chile and Argentina). It is the only extant species in the ancient order Microbiotheria, and the sole New... - Magnorder AmeridelphiaAmeridelphiaAmeridelphia is traditionally a superorder that includes all marsupials living in the Americas except for the Monito del Monte...
: New World marsupials
- Magnorder Australidelphia
- Cohort Placentalia: placentals
- Magnorder XenarthraXenarthraThe superorder Xenarthra is a group of placental mammals , existent today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced back as far as the Paleogene in South America...
: xenarthrans - Magnorder EpitheriaEpitheriaEpitherians comprise all the placental mammals except the Xenarthra. They are primarily characterized by having a stirrup-shaped stapes in the middle ear, which allows for passage of a blood vessel. This is in contrast to the column-shaped stapes found in marsupials, monotremes, and xenarthrans...
: epitheres- Grandorder Anagalida: lagomorphsLagomorphaThe lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families, the Leporidae , and the Ochotonidae...
, rodentRodentRodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
s, and elephant shrewElephant shrewElephant shrews or jumping shrews are small insectivorous mammals native to Africa, belonging to the family Macroscelididae, in the order Macroscelidea...
s - Grandorder FeraeFeraeFerae is a clade of mammals, consisting of the orders Carnivora and Pholidota . Pangolins do not look much like carnivorans , and were thought to be the closest relatives of Xenarthra...
: carnivoraCarnivoraThe diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...
ns, pangolinPangolinA pangolin , also scaly anteater or Trenggiling, is a mammal of the order Pholidota. There is only one extant family and one genus of pangolins, comprising eight species. There are also a number of extinct taxa. Pangolins have large keratin scales covering their skin and are the only mammals with...
s, †creodonts, and relatives - Grandorder Lipotyphla: insectivoraInsectivoraThe order Insectivora is a now-abandoned biological grouping within the class of mammals...
ns - Grandorder ArchontaArchontaThe Archonta are a group of mammals considered a superorder in some classifications.The Archonta consist of the following orders:*Primates*Plesiadapiformes *Scandentia *Dermoptera *Chiroptera...
: batBatBats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
s, primatePrimateA primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
s, colugoColugoColugos are arboreal gliding mammals found in South-east Asia. There are just two extant species, which make up the entire family Cynocephalidae and order Dermoptera. They are the most capable of all gliding mammals, using flaps of extra skin between their legs to glide from higher to lower...
s, and treeshrewTreeshrewThe treeshrews are small mammals native to the tropical forests of Southeast Asia. They make up the families Tupaiidae, the treeshrews, and Ptilocercidae, the pen-tailed treeshrews, and the entire order Scandentia. There are 20 species in 5 genera...
s - Grandorder Ungulata: ungulates
- Order Tubulidentata incertae sedisIncertae sedis, is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any...
: aardvarkAardvarkThe aardvark is a medium-sized, burrowing, nocturnal mammal native to Africa... - Mirorder Eparctocyona: †condylarthCondylarthCondylarthra is an order of extinct placental mammals known primarily from the Paleocene and Eocene epochs. Condylarths are among the most characteristic Paleocene mammals and they illustrate the evolutionary level of the Paleocene mammal fauna....
s, whaleWhaleWhale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
s, and artiodactyls (even-toed ungulates) - Mirorder †MeridiungulataMeridiungulataMeridiungulata is an extinct clade with the rank of cohort or super-order, containing the South-American ungulates: Pyrotheria , Astrapotheria, Notoungulata and Litopterna...
: South American ungulates - Mirorder Altungulata: perissodactyls (odd-toed ungulates), elephantElephantElephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...
s, manateeManateeManatees are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows...
s, and hyraxHyraxA hyrax is any of four species of fairly small, thickset, herbivorous mammals in the order Hyracoidea. The rock hyrax Procavia capensis, the yellow-spotted rock hyrax Heterohyrax brucei, the western tree hyrax Dendrohyrax dorsalis, and the southern tree hyrax, Dendrohyrax arboreus live in Africa...
es
- Order Tubulidentata incertae sedis
- Grandorder Anagalida: lagomorphs
- Magnorder Xenarthra
- Cohort Marsupialia: marsupials
- Supercohort Theria
- Infraclass †Allotheria
Molecular classification of placentals
Molecular studies based on DNADNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
analysis have suggested new relationships among mammal families over the last few years. Most of these findings have been independently validated by retrotransposon
Retrotransposon
Retrotransposons are genetic elements that can amplify themselves in a genome and are ubiquitous components of the DNA of many eukaryotic organisms. They are a subclass of transposon. They are particularly abundant in plants, where they are often a principal component of nuclear DNA...
presence/absence data
Retrotransposon Marker
Retrotransposon markers are retrotransposons that are used as cladistic markers.The analysis of SINEs – Short INterspersed Elements – LINEs – Long INterspersed Elements – or truncated LTRs – Long Terminal Repeats – as molecular cladistic markers represents a particularly interesting complement to...
. The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals. Molecular clock
Molecular clock
The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...
s suggest that these clades diverged from early common ancestors in the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
, but fossils have not yet been found to corroborate this hypothesis. These molecular findings are consistent with mammal zoogeography
Zoogeography
Zoogeography is the branch of the science of biogeography that is concerned with the geographic distribution of animal species.-External links:*: A course outline and collection of Web resources by Dr. Taylor, UBC...
:
Following molecular DNA sequence analyses, the first divergence was that of the Afrotheria
Afrotheria
Afrotheria is a clade of mammals, the living members of which belong to groups from Africa or of African origin: golden moles, sengis , tenrecs, aardvarks, hyraxes, elephants and sea cows. The common ancestry of these animals was not recognized until the late 1990s...
110–100 million years ago. The Afrotheria proceeded to evolve and diversify in the isolation of the African-Arabian continent. The Xenarthra
Xenarthra
The superorder Xenarthra is a group of placental mammals , existent today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced back as far as the Paleogene in South America...
, isolated in South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, diverged from the Boreoeutheria
Boreoeutheria
Boreoeutheria is a clade of placental mammals that is composed of the sister taxa Laurasiatheria and Euarchontoglires...
approximately 100–95 million years ago. According to an alternative view, the Xenarthra has the Afrotheria as closest allies, forming the Atlantogenata
Atlantogenata
Atlantogenata is a proposed clade of mammals containing the cohorts or super-orders Afrotheria, Xenarthra and Meridiungulata. These groups originated and radiated in the South American and African continents, presumably in the Cretaceous. Together with Boreoeutheria it makes up Eutheria...
as sistergroup to Boreoeutheria. The Boreoeutheria split into the Laurasiatheria
Laurasiatheria
Laurasiatheria is a large group of placental mammals believed to have originated on the northern supercontinent of Laurasia. It includes shrews, hedgehogs, pangolins, bats, whales, most hoofed mammals, and carnivorans, among others....
and Euarchontoglires
Euarchontoglires
Euarchontoglires is a clade of mammals, the living members of which are rodents, lagomorphs, treeshrews, colugos and primates .-Evolutionary relationships:...
between 95 and 85 mya; both of these groups evolved on the northern continent of Laurasia
Laurasia
In paleogeography, Laurasia was the northernmost of two supercontinents that formed part of the Pangaea supercontinent from approximately...
. After tens of millions of years of relative isolation, Africa-Arabia collided with Eurasia, exchanging Afrotheria and Boreoeutheria. The formation of the Isthmus of Panama
Isthmus of Panama
The Isthmus of Panama, also historically known as the Isthmus of Darien, is the narrow strip of land that lies between the Caribbean Sea and the Pacific Ocean, linking North and South America. It contains the country of Panama and the Panama Canal...
linked South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
and North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, which facilitated the exchange of mammal species in the Great American Interchange
Great American Interchange
The Great American Interchange was an important paleozoogeographic event in which land and freshwater fauna migrated from North America via Central America to South America and vice versa, as the volcanic Isthmus of Panama rose up from the sea floor and bridged the formerly separated continents...
. The traditional view that no placental mammals reached Australasia
Australasia
Australasia is a region of Oceania comprising Australia, New Zealand, the island of New Guinea, and neighbouring islands in the Pacific Ocean. The term was coined by Charles de Brosses in Histoire des navigations aux terres australes...
until about 5 million years ago when bats and murine
Murinae
The Old World rats and mice, part of the subfamily Murinae in the family Muridae, comprise at least 519 species. This subfamily is larger than all mammal families except the Cricetidae and Muridae, and is larger than all mammal orders except the bats and the remainder of the...
rodents arrived has been challenged by recent evidence and may need to be reassessed. These molecular results are still controversial because they are not reflected by morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
data, and thus not accepted by many systematists. Further there is some indication from retrotransposon presence/absence data that the traditional Epitheria
Epitheria
Epitherians comprise all the placental mammals except the Xenarthra. They are primarily characterized by having a stirrup-shaped stapes in the middle ear, which allows for passage of a blood vessel. This is in contrast to the column-shaped stapes found in marsupials, monotremes, and xenarthrans...
hypothesis, suggesting Xenarthra
Xenarthra
The superorder Xenarthra is a group of placental mammals , existent today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced back as far as the Paleogene in South America...
as the first divergence, might be true. With the old order Insectivora
Insectivora
The order Insectivora is a now-abandoned biological grouping within the class of mammals...
shown to be polyphylectic
Polyphyly
A polyphyletic group is one whose members' last common ancestor is not a member of the group.For example, the group consisting of warm-blooded animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded...
and more properly subdivided (as Afrosoricida
Afrosoricida
The order Afrosoricida contains the golden moles of southern Africa and the tenrecs of Madagascar and Africa, two families of small mammals that have traditionally been considered to be a part of the order Insectivora.Some biologists use Tenrecomorpha as the name for the tenrec-golden mole clade,...
, Erinaceomorpha
Erinaceidae
Erinaceidae is the only living family in the order Erinaceomorpha, which has recently been subsumed with Soricomorpha into the order Eulipotyphla...
, and Soricomorpha
Soricomorpha
The order Soricomorpha is taxon within the class of mammals. In previous years it formed a significant group within the former order Insectivora...
), the following classification for placental mammals contains 21 orders:
- Clade AtlantogenataAtlantogenataAtlantogenata is a proposed clade of mammals containing the cohorts or super-orders Afrotheria, Xenarthra and Meridiungulata. These groups originated and radiated in the South American and African continents, presumably in the Cretaceous. Together with Boreoeutheria it makes up Eutheria...
- Group I: AfrotheriaAfrotheriaAfrotheria is a clade of mammals, the living members of which belong to groups from Africa or of African origin: golden moles, sengis , tenrecs, aardvarks, hyraxes, elephants and sea cows. The common ancestry of these animals was not recognized until the late 1990s...
- Clade AfroinsectiphiliaAfroinsectiphiliaThe Afroinsectiphilia is a clade that has been proposed based on the results of recent molecular studies. Many of its orders were once regarded as part of the order Insectivora, but Insectivora is now considered to be polyphyletic and is, as a result, possibly obsolete...
- Order Macroscelidea: elephant shrews (Africa)
- Order AfrosoricidaAfrosoricidaThe order Afrosoricida contains the golden moles of southern Africa and the tenrecs of Madagascar and Africa, two families of small mammals that have traditionally been considered to be a part of the order Insectivora.Some biologists use Tenrecomorpha as the name for the tenrec-golden mole clade,...
: tenrecs and golden moles (Africa) - Order Tubulidentata: aardvark (Africa south of the Sahara)
- Clade PaenungulataPaenungulataPaenungulata is a taxon that groups some remarkable mammals, including three orders that are extant: Proboscidea , Sirenia , and Hyracoidea . At least two more orders are known only as fossils, namely Embrithopoda and Desmostylia...
- Order Hyracoidea: hyraxes or dassies (Africa, Arabia)
- Order ProboscideaProboscideaProboscidea is a taxonomic order containing one living family, Elephantidae, and several extinct families. This order was first described by J. Illiger in 1881 and encompasses the trunked mammals...
: elephants (Africa, Southeast Asia) - Order SireniaSireniaSirenia is an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. Four species are living, in two families and genera. These are the dugong and manatees...
: dugong and manatees (cosmopolitan tropical)
- Clade Afroinsectiphilia
- Group II: XenarthraXenarthraThe superorder Xenarthra is a group of placental mammals , existent today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced back as far as the Paleogene in South America...
- Order PilosaPilosaThe order Pilosa is a group of placental mammals, extant today only in the Americas. It includes the anteaters and sloths, including the recently extinct ground sloths....
: sloths and anteaters (Neotropical) - Order CingulataArmadilloArmadillos are New World placental mammals, known for having a leathery armor shell. Dasypodidae is the only surviving family in the order Cingulata, part of the superorder Xenarthra along with the anteaters and sloths. The word armadillo is Spanish for "little armored one"...
: armadillos (Americas)
- Order Pilosa
- Group I: Afrotheria
- Clade BoreoeutheriaBoreoeutheriaBoreoeutheria is a clade of placental mammals that is composed of the sister taxa Laurasiatheria and Euarchontoglires...
- Group III: EuarchontogliresEuarchontogliresEuarchontoglires is a clade of mammals, the living members of which are rodents, lagomorphs, treeshrews, colugos and primates .-Evolutionary relationships:...
(Supraprimates)- Superorder EuarchontaEuarchontaThe Euarchonta are a grandorder of mammals containing four orders: the Dermoptera or colugos, the Scandentia or treeshrews, the extinct Plesiadapiformes, and the Primates....
- Order Scandentia: treeshrews (Southeast Asia).
- Order Dermoptera: flying lemurs or colugos (Southeast Asia)
- Order PrimatePrimateA primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
s: lemurs, bushbabies, monkeys, apes, human (cosmopolitan)
- Superorder GliresGliresGlires is a clade consisting of rodents and lagomorphs . This hypothesis that these form a monophyletic group has been long debated based on morphological evidence, although recent morphological studies strongly support monophyly of Glires...
- Order LagomorphaLagomorphaThe lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families, the Leporidae , and the Ochotonidae...
: pikas, rabbits, hares (Eurasia, Africa, Americas) - Order RodentRodentRodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
ia: rodents (cosmopolitan)
- Order Lagomorpha
- Superorder Euarchonta
- Group IV: LaurasiatheriaLaurasiatheriaLaurasiatheria is a large group of placental mammals believed to have originated on the northern supercontinent of Laurasia. It includes shrews, hedgehogs, pangolins, bats, whales, most hoofed mammals, and carnivorans, among others....
- Order ErinaceomorphaErinaceidaeErinaceidae is the only living family in the order Erinaceomorpha, which has recently been subsumed with Soricomorpha into the order Eulipotyphla...
: hedgehogs - Order SoricomorphaSoricomorphaThe order Soricomorpha is taxon within the class of mammals. In previous years it formed a significant group within the former order Insectivora...
: moles, shrews, solenodons - Clade FerungulataFerungulataFerungulata is traditionally a clade with the rank of cohort within the placental mammals. Established by George Gaylord Simpson in 1945, it includes the Carnivora, Perissodactyla and Artiodactyla as well as Tubulidentata and a superorder, Paenungulata, plus a number of orders known only as fossils...
- Clade CetartiodactylaCetartiodactylaCetartiodactyla is the clade in which whales and even-toed ungulates have currently been placed. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. The term Cetartiodactyla reflects the idea that whales evolved within the artiodactyls...
- Order CetaceaCetaceaThe order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...
: whales, dolphins and porpoises - Order Artiodactyla: even-toed ungulates, including pigPigA pig is any of the animals in the genus Sus, within the Suidae family of even-toed ungulates. Pigs include the domestic pig, its ancestor the wild boar, and several other wild relatives...
s, hippopotamusHippopotamusThe hippopotamus , or hippo, from the ancient Greek for "river horse" , is a large, mostly herbivorous mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae After the elephant and rhinoceros, the hippopotamus is the third largest land mammal and the heaviest...
, camelCamelA camel is an even-toed ungulate within the genus Camelus, bearing distinctive fatty deposits known as humps on its back. There are two species of camels: the dromedary or Arabian camel has a single hump, and the bactrian has two humps. Dromedaries are native to the dry desert areas of West Asia,...
s, giraffeGiraffeThe giraffe is an African even-toed ungulate mammal, the tallest of all extant land-living animal species, and the largest ruminant...
, deerDeerDeer are the ruminant mammals forming the family Cervidae. Species in the Cervidae family include white-tailed deer, elk, moose, red deer, reindeer, fallow deer, roe deer and chital. Male deer of all species and female reindeer grow and shed new antlers each year...
, antelopeAntelopeAntelope is a term referring to many even-toed ungulate species indigenous to various regions in Africa and Eurasia. Antelopes comprise a miscellaneous group within the family Bovidae, encompassing those old-world species that are neither cattle, sheep, buffalo, bison, nor goats...
, cattleCattleCattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius...
, sheep, goatGoatThe domestic goat is a subspecies of goat domesticated from the wild goat of southwest Asia and Eastern Europe. The goat is a member of the Bovidae family and is closely related to the sheep as both are in the goat-antelope subfamily Caprinae. There are over three hundred distinct breeds of...
s
- Order Cetacea
- Clade PegasoferaePegasoferaePegasoferae is a proposed clade of mammals based on genomic research in molecular systematics by Nishihara, Hasegawa and Okada .To the surprise of the authors, their data led them to propose a clade that includes bats , carnivores such as cats and dogs , horses and other odd-toed ungulates and...
- Order Chiroptera: bats (cosmopolitan)
- Clade ZooamataZooamataZooamata is a proposed clade of mammals consisting of Ferae and Perissodactyla . Together with Cetartiodactyla and chiroptera it forms Scrotifera , and is part of Laurasiatheria...
- Order Perissodactyla: odd-toed ungulates, including horseHorseThe horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...
s, donkeyDonkeyThe donkey or ass, Equus africanus asinus, is a domesticated member of the Equidae or horse family. The wild ancestor of the donkey is the African Wild Ass, E...
s, zebraZebraZebras are several species of African equids united by their distinctive black and white stripes. Their stripes come in different patterns unique to each individual. They are generally social animals that live in small harems to large herds...
s, tapirTapirA Tapir is a large browsing mammal, similar in shape to a pig, with a short, prehensile snout. Tapirs inhabit jungle and forest regions of South America, Central America, and Southeast Asia. There are four species of Tapirs: the Brazilian Tapir, the Malayan Tapir, Baird's Tapir and the Mountain...
s, and rhinocerosRhinocerosRhinoceros , also known as rhino, is a group of five extant species of odd-toed ungulates in the family Rhinocerotidae. Two of these species are native to Africa and three to southern Asia....
es - Clade FeraeFeraeFerae is a clade of mammals, consisting of the orders Carnivora and Pholidota . Pangolins do not look much like carnivorans , and were thought to be the closest relatives of Xenarthra...
- Order Pholidota: pangolins or scaly anteaters (Africa, South Asia)
- Order CarnivoraCarnivoraThe diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...
: carnivores (cosmopolitan)
- Order Perissodactyla: odd-toed ungulates, including horse
- Clade Cetartiodactyla
- Order Erinaceomorpha
- Group III: Euarchontoglires
Evolutionary history
Synapsida, the group which contains mammals and their extinct relatives, originated during the Pennsylvanian
Pennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...
epoch
Epoch (geology)
An epoch is a subdivision of the geologic timescale based on rock layering. In order, the higher subdivisions are periods, eras and eons. We are currently living in the Holocene epoch...
, when they split from the lineage that led to reptiles and birds. Non-mammalian synapsids were once called "mammal-like reptiles", although they are usually no longer considered reptiles. Mammals evolved from non-mammalian synapsids during the Early Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
.
Evolution from amniotes in the Paleozoic
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s were amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
s. Like their amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
predecessors, they have lungs and limbs. Amniotes' eggs, however, have internal membranes which allow the developing embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
to breathe but keep water in. Hence amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water.
The first amniotes apparently arose in the late
Pennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...
Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
. They descended from earlier reptiliomorph
Reptiliomorpha
Reptiliomorpha refers to an order or subclass of reptile-like amphibians, which gave rise to the amniotes in the Carboniferous. Under phylogenetic nomenclature, the Reptiliomorpha includes their amniote descendants though, even in phylogenetic nomenclature, the name is mostly used when referring to...
amphibians, which lived on land already inhabited by insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s and other invertebrates, and by fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
s, moss
Moss
Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
es and other plants. Within a few million years two important amniote lineages became distinct: the synapsid
Synapsid
Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...
s, which include mammals; and the sauropsids, which include lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...
s, snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...
s, crocodilians, dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s. Synapsids have a single hole (temporal fenestra) low on each side of the skull.
One synapsid group, the pelycosaur
Pelycosaur
The pelycosaurs are an informal grouping composed of basal or primitive Late Paleozoic synapsid amniotes. Some species were quite large and could grow up to 3 meters or more, although most species were much smaller...
s, were the most common land vertebrates of the early Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
and included the largest land animals of the time.
Therapsids descended from pelycosaurs in the middle Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
, about 260M years ago, and took over their position as the dominant land vertebrates. They differ from pelycosaurs in several features of the skull and jaws, including: larger temporal fenestrae and incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s which are equal in size. The therapsids went through a series of stages, beginning with animals which were very like their pelycosaur ancestors and ending with the Triassic cynodont
Cynodont
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
s, some of which could easily be mistaken for mammals. Those stages were characterized by:
- gradual development of a bony secondary palateHard palateThe hard palate is a thin horizontal bony plate of the skull, located in the roof of the mouth. It spans the arch formed by the upper teeth.It is formed by the palatine process of the maxilla and horizontal plate of palatine bone....
. - progress towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraintCarrier's constraintCarrier's constraint is the observation that air-breathing vertebrates which have two lungs and flex their bodies sideways during locomotion find it very difficult to move and breathe at the same time, because:...
. But this process was slow and erratic – for example: all herbivorous non-mammaliaform therapsids retained sprawling limbs (some late forms may have had semi-erect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semi-sprawling hindlimbs. In fact modern monotremeMonotremeMonotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s still have semi-sprawling limbs. - the dentary gradually becoming the main bone of the lower jaw; and in the TriassicTriassicThe Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
, progress towards the fully mammalian jaw (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of Reptiles) - there is possible evidence of hair in Triassic therapsids, but none for Permian therapsids.
- some scientists have argued that some Triassic therapsids show signs of lactationLactationLactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...
.
True mammals evolve in the Triassic
The Permian–Triassic extinction event, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of the therapsids. In the Early Triassic all the medium to large land animal niches were taken over by early archosaurs, which over an extended period of time (35 million years) evolved into crocodilians, pterosaurPterosaur
Pterosaurs were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period . Pterosaurs are the earliest vertebrates known to have evolved powered flight...
s, dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s. After this "Triassic Takeover" the cynodonts and their descendants
Lineal descendant
A lineal descendant, in legal usage, refers to a blood relative in the direct line of descent. The children, grandchildren, great-grandchildren, etc...
could only survive as small, mainly nocturnal insectivore
Insectivore
An insectivore is a type of carnivore with a diet that consists chiefly of insects and similar small creatures. An alternate term is entomophage, which also refers to the human practice of eating insects....
s.
The first true mammals appeared in the Late Triassic (ca. 200 million years ago), over 70 million years after the first therapsids and approximately 30 million years after the first mammaliaformes. The rat-like Hadrocodium
Hadrocodium
Hadrocodium wui is an extinct basal mammal species that lived during the Lower Jurassic in what is now the Yunnan province of China...
appears to be in the middle of the transition to true mammal status — it had a mammalian jaw joint (formed by the dentary and squamosal bones), but there is some debate about whether its middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
was fully mammalian. The majority of the mammal species that existed in the Mesozoic Era
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
were characterized by Multituberculates.
The earliest known monotreme
Monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
is Teinolophos
Teinolophos
Teinolophos trusleri was a prehistoric species of monotreme, or egg-laying mammal. It is known from a lower jawbone found in Flat Rocks, Victoria, Australia. It lived during the Aptian age of the Lower Cretaceous. It is the earliest known relative of the Platypus.The species name honours the...
, which lived about 123M years ago in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. Monotremes have some features which may be inherited from the original amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
s:
- they use the same orifice to urinate, defecate and reproduce ("monotreme" means "one hole") – as lizards and birds also do.
- they lay eggsEgg (biology)An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
which are leathery and uncalcified, like those of lizards, turtles and crocodilians.
Unlike other mammals, female monotremes do not have nipples and feed their young by "sweating" milk from patches on their bellies.
The oldest known marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
is Sinodelphys
Sinodelphys
Sinodelphys is an extinct mammal from the Early Cretaceous. To date, it is the oldest metatherian fossil known, estimated to be 125 million years old...
, found in 125M-year old early Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
("true beasts") are all placentals. An early eutherian, Eomaia
Eomaia
Eomaia is an extinct fossil mammal, discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower Cretaceous about . The fossil is in length and virtually complete. An estimate of the body weight is between . It is exceptionally...
, found in China and dated to 125M years ago, obtained some features which are more like those of marsupials, which suggested it was perhaps a transitional fossil that eventually gave rise to the placental lineage (the surviving metatherians):
- Epipubic bonesEpipubic bonesEpipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials and of some fossil mammals: multituberculates, monotremes, and even basal eutherians ....
extending forwards from the pelvis, which are not found in any modern placental, but are found in marsupials, monotremeMonotremeMonotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s and mammaliformes such as multituberculates. In other words, they appear to be an ancestral feature which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles of these animals during locomotion, reducing the amount of space being presented, which placentals require to contain their fetusFetusA fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
during gestation periods. - A narrow pelvic outlet, which indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.
Now the discovery of a partial skeleton of a small, shrewlike mammal, described online in Nature in August of 2011, pushes back the date of the divergence by 30 million years, to 160 million years ago. Found in the famous fossil beds of Liaoning, China, the newly discovered little mammal has been named Juramaia sinensis, or "Jurassic mother from China."
It is not certain when true placental mammals evolved – the earliest undisputed fossils of placentals come from the early Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
, after the extinction of the dinosaurs.
Mammals and near-mammals expanded out of their nocturnal insectivore niche from the mid Jurassic onwards – for example Castorocauda had adaptations for swimming, digging and catching fish.
Rise to dominance in the Cenozoic
Mammals took over the medium- to large-sized ecological niches in the CenozoicCenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
, after the Cretaceous–Tertiary extinction event emptied ecological space once filled by reptiles. Then mammals diversified very quickly, both birds and mammals show an exponential rise in diversity. An example is that the earliest known bat dates from about 50M years ago, only 15M years after the extinction of the dinosaurs.
Recent molecular phylogenetic studies suggest that most placental orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
diverged about 100M to 85M years ago, but that modern families
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
first appeared in the late Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
and early Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
But paleontologists object that no placental fossils have been found from before the end of the Cretaceous.
During the Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
several groups of mammals appeared which were much larger than their nearest modern equivalents – but none was even close to the size of the largest dinosaurs with similar feeding habits.
Earliest appearances of features
HadrocodiumHadrocodium
Hadrocodium wui is an extinct basal mammal species that lived during the Lower Jurassic in what is now the Yunnan province of China...
, whose fossils date from the early Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
(approx. 195 million years ago), provides the first clear evidence of fully mammalian jaw joints.
It has been suggested that the original function of lactation (milk production) was to keep eggs moist. Much of the argument is based on monotremes (egg-laying mammals).
The earliest clear evidence of hair or fur is in fossils of Castorocauda, from 164M years ago in the mid Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
. From 1955 onwards some scientists have interpreted the foramina (passages) in the maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
e (upper jaws) and premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
e (small bones in front of the maxillae) of cynodont
Cynodont
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
s as channels which supplied blood vessels and nerves to vibrissae (whiskers), and suggested that this was evidence of hair or fur. But foramina do not necessarily show that an animal had vibrissae – for example the modern lizard Tupinambis has foramina which are almost identical to those found in the non-mammalian cynodont Thrinaxodon
Thrinaxodon
Thrinaxodon was a cynodont, an ermine-sized therapsid. Pits on the skull suggest that Thrinaxodon may have had whiskers, and by extension a protective covering of fur. There are suggestions that it was warm-blooded...
.
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
(distinguishing characteristic) of the Boreosphenida
Mammal classification
Mammalia is a class of animal within the Phylum Chordata. Mammal classification has been through several iterations since Carolus Linnaeus initially defined the class. Many earlier ideas have been completely abandoned by modern taxonomists, among these are the idea that bats are related to birds...
, a group which contains the Theria
Theria
Theria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....
and therefore includes the last common ancestor of modern marsupial and placentals – and therefore that all earlier mammals had sprawling limbs. Sinodelphys
Sinodelphys
Sinodelphys is an extinct mammal from the Early Cretaceous. To date, it is the oldest metatherian fossil known, estimated to be 125 million years old...
(the earliest known marsupial) and Eomaia
Eomaia
Eomaia is an extinct fossil mammal, discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower Cretaceous about . The fossil is in length and virtually complete. An estimate of the body weight is between . It is exceptionally...
(the earliest known eutheria
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
n) lived about 125M years ago, so erect limbs must have evolved before then.
It is currently very difficult to be confident when endothermy first appeared in the evolution of mammals. Modern monotremes have a lower body temperature and more variable metabolic rate than marsupials and placentals. So the main question is when a monotreme-like metabolism evolved in mammals. The evidence found so far suggests Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
cynodont
Cynodont
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
s may have had fairly high metabolic rates, but is not conclusive. In particular it is difficult to see how small animals can maintain a high and stable body temperature without fur.
Skeletal system
The majority of mammals have seven cervical vertebraeCervical vertebrae
In vertebrates, cervical vertebrae are those vertebrae immediately inferior to the skull.Thoracic vertebrae in all mammalian species are defined as those vertebrae that also carry a pair of ribs, and lie caudal to the cervical vertebrae. Further caudally follow the lumbar vertebrae, which also...
(bones in the neck
Neck
The neck is the part of the body, on many terrestrial or secondarily aquatic vertebrates, that distinguishes the head from the torso or trunk. The adjective signifying "of the neck" is cervical .-Boner anatomy: The cervical spine:The cervical portion of the human spine comprises seven boney...
); this includes bat
Bat
Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
s, giraffe
Giraffe
The giraffe is an African even-toed ungulate mammal, the tallest of all extant land-living animal species, and the largest ruminant...
s, whale
Whale
Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
s, and human
Human
Humans are the only living species in the Homo genus...
s. The few exceptions include the manatee
Manatee
Manatees are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows...
and the two-toed sloth
Two-toed sloth
Choloepus is a genus of mammals of Central and South America, within the family Megalonychidae consisting of two-toed sloths. There are only two species of Choloepus : Linnaeus's Two-toed Sloth and Hoffmann's Two-toed Sloth...
, which have only six cervical vertebrae, and the three-toed sloth
Three-toed sloth
The three-toed sloths are tree-living mammals from South and Central America. They are the only members of the genus Bradypus and the family Bradypodidae. There are four living species of three-toed sloths...
with nine cervical vertebrae.
Respiratory system
The lungs of mammals have a spongy texture and are honeycombed with epitheliumEpithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans
Human lung
The human lungs are the organs of respiration in humans. Humans have two lungs, with the left being divided into two lobes and the right into three lobes. Together, the lungs contain approximately of airways and 300 to 500 million alveoli, having a total surface area of about in...
are typical of this type of lung.
Breathing is largely driven by the muscular diaphragm which divides the thorax from the abdominal cavity, forming a dome with its convexity towards the thorax. Contraction of the diaphragm flattens the dome increasing the volume of the cavity in which the lung is enclosed. Air enters through the oral and nasal cavities; it flows through the larynx, trachea and bronchi and expands the alveoli. Relaxation of the diaphragm has the opposite effect, passively recoiling during normal breathing. During exercise, the abdominal wall contracts
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...
, increasing visceral pressure on the diaphragm, thus forcing the air out more quickly and forcefully. The rib cage
Rib cage
The rib cage is an arrangement of bones in the thorax of animals. It is formed by the vertebral column, ribs and sternum and encloses the heart and lungs....
itself also is able to expand and contract the thoracic cavity to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its pressure gradient. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows
Bellows
A bellows is a device for delivering pressurized air in a controlled quantity to a controlled location.Basically, a bellows is a deformable container which has an outlet nozzle. When the volume of the bellows is decreased, the air escapes through the outlet...
. Mammals take oxygen into their lungs, and discard carbon dioxide.
Nervous system
All mammalian brains possess a neocortexNeocortex
The neocortex , also called the neopallium and isocortex , is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI...
, a brain region that is unique to mammals. Placental mammals have a corpus callosum
Corpus callosum
The corpus callosum , also known as the colossal commissure, is a wide, flat bundle of neural fibers beneath the cortex in the eutherian brain at the longitudinal fissure. It connects the left and right cerebral hemispheres and facilitates interhemispheric communication...
unlike monotremes and marsupials. The size and number of cortical areas (Brodmann
Brodmann
Brodmann may refer to:* Korbinian Brodmann, German neurologist* Brodmann area, a region in the brain cortex...
's areas) is least in monotremes (about 8-10) and most in placentals (up to 50).
Integumentary system
The integumentary systemSkin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
is made up of three layers: the outermost epidermis, the dermis
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...
, and the hypodermis
Hypodermis
The hypodermis, also called the hypoderm, subcutaneous tissue, or superficial fascia is the lowermost layer of the integumentary system in vertebrates. Types of cells that are found in the hypodermis are fibroblasts, adipose cells, and macrophages...
.
The epidermis is typically ten to thirty cells thick, its main function being to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is fifteen to forty times thicker than the epidermis. The dermis is made up of many components such as bony structures and blood vessels. The hypodermis is made up of adipose tissue
Adipose tissue
In histology, adipose tissue or body fat or fat depot or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts...
. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species.
Although mammals and other animals have cilia that superficially may resemble it, no other animals except mammals have hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
. It is a definitive characteristic of the class. Some mammals have very little, but nonetheless, careful examination reveals the characteristic, often in obscure parts of their bodies. None are known to have hair that naturally is blue or green in color although some cetaceans, along with the mandrill
Mandrill
The mandrill is a primate of the Old World monkey family, closely related to the baboons and even more closely to the drill. Both the mandrill and the drill were once classified as baboons in genus Papio, but recent research has determined they should be separated into their own genus, Mandrillus...
s, appear to have shades of blue skin. Many mammals are indicated as having blue hair or fur, but in all known cases, it has been found to be a shade of gray. The two-toed sloth
Two-toed sloth
Choloepus is a genus of mammals of Central and South America, within the family Megalonychidae consisting of two-toed sloths. There are only two species of Choloepus : Linnaeus's Two-toed Sloth and Hoffmann's Two-toed Sloth...
and the polar bear
Polar Bear
The polar bear is a bear native largely within the Arctic Circle encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is the world's largest land carnivore and also the largest bear, together with the omnivorous Kodiak Bear, which is approximately the same size...
may seem to have green fur, but this color is caused by algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
growths.
Reproductive system
Vivipary
Vivipary has two different meanings. In animals, it means development of the embryo inside the body of the mother, eventually leading to live birth, as opposed to laying eggs...
), but a few, namely the monotremes, lay eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
. The platypus
Platypus
The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
and the echidna
Echidna
Echidnas , also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs...
present a particular sex determination system
Sex-determination system
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual...
that is different from other vertebrates.
Certain glands of mammals known as mammary gland
Mammary gland
A mammary gland is an organ in mammals that produces milk to feed young offspring. Mammals get their name from the word "mammary". In ruminants such as cows, goats, and deer, the mammary glands are contained in their udders...
s are specialized to produce milk
Milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the nipple
Nipple
In its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts or udder of a mammal by which breast milk is delivered to a mother's young. In this sense, it is often called a teat, especially when referring to non-humans, and...
s seen in most mammals, but they do have mammary glands.
Viviparous mammals are classified into the subclass Theria and are divided into two infraclasses: Metatheria
Metatheria
Metatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupialia though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals.The earliest known...
(of which only the Marsupialia survive), and Eutheria. Marsupialia, or marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s, have short gestation
Gestation
Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....
periods and give birth to undeveloped young which are contained within a pouch-like sac, (marsupium
Pouch (marsupial)
The pouch is a distinguishing feature of female marsupials ; the name marsupial is derived from the Latin marsupium, meaning "pouch". Marsupials give birth to a live but relatively undeveloped fetus called a joey. When the joey is born it crawls from inside the mother to the pouch...
), located in the front of the mothers' abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
. Eutherians, commonly known as placentals, are mammals that give birth to complete and fully developed young. This is usually characterized by long gestation
Gestation
Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....
periods. The majority of mammal species are classified as eutherians.
Endothermy
Nearly all mammals are endothermic ("warm-bloodedWarm-blooded
The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...
"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in cold weather and climates where non-avian reptiles and large insects cannot.
Endothermy requires plenty of food energy, so pound for pound mammals eat more food than most reptiles. Small insectivorous mammals eat prodigious amounts for their size.
A rare exception, the naked mole rat
Naked Mole Rat
The naked mole rat , also known as the sand puppy or desert mole rat, is a burrowing rodent native to parts of East Africa and the only species currently classified in the genus Heterocephalus...
produces little metabolic heat, so it is considered an operational poikilotherm
Poikilotherm
A poikilotherm is an organism whose internal temperature varies considerably. It is the opposite of a homeotherm, an organism which maintains thermal homeostasis. Usually the variation is a consequence of variation in the ambient environmental temperature...
. Bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s are also endothermic, so endothermy is not a defining mammalian feature.
Intelligence
Intelligence
Intelligence has been defined in different ways, including the abilities for abstract thought, understanding, communication, reasoning, learning, planning, emotional intelligence and problem solving....
itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rats
RATS
RATS may refer to:* RATS , Regression Analysis of Time Series, a statistical package* Rough Auditing Tool for Security, a computer program...
, for example, are considered to be highly intelligent as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat
Biome
Biomes are climatically and geographically defined as similar climatic conditions on the Earth, such as communities of plants, animals, and soil organisms, and are often referred to as ecosystems. Some parts of the earth have more or less the same kind of abiotic and biotic factors spread over a...
. In some mammals, food gathering appears to be related to intelligence: a deer
Deer
Deer are the ruminant mammals forming the family Cervidae. Species in the Cervidae family include white-tailed deer, elk, moose, red deer, reindeer, fallow deer, roe deer and chital. Male deer of all species and female reindeer grow and shed new antlers each year...
feeding on plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s has a brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
smaller than a cat
Cat
The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...
, which must think to outwit its prey.
Locomotion
Mammals evolved from four-legged ancestors. They use their limbs to walk, climb, swim, and fly. Some land mammals have toes that produce claws and hooves for climbing and running. Aquatic mammals such as whales and dolphins have flippers which evolved from legs.Arboreal
SlothSloth
Sloths are the six species of medium-sized mammals belonging to the families Megalonychidae and Bradypodidae , part of the order Pilosa and therefore related to armadillos and anteaters, which sport a similar set of specialized claws.They are arboreal residents of the jungles of Central and South...
s travel slowly along branches rather than swinging energetically like monkeys.
Aquatic
Buoyed by their aquatic environment, whales have evolved into the largest mammals and indeed the largest animals ever.Feeding
To maintain a high constant body temperature is energy expensive – mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different speciesSpecies
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
have since adapted to meet their dietary requirements in a variety of ways. Some eat other animals – this is a carnivorous
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...
diet (and includes insectivorous
Insectivore
An insectivore is a type of carnivore with a diet that consists chiefly of insects and similar small creatures. An alternate term is entomophage, which also refers to the human practice of eating insects....
diets). Other mammals, called herbivores, eat plants. A herbivorous diet includes sub-types such as fruit-eating and grass-eating. An omnivore
Omnivore
Omnivores are species that eat both plants and animals as their primary food source...
eats both prey and plants. Carnivorous mammals have a simple digestive tract, because the proteins, lipids, and minerals found in meat require little in the way of specialized digestion. Plants, on the other hand, contain complex carbohydrates, such as cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
. The digestive tract of an herbivore is therefore host to bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that ferment these substances, and make them available for digestion. The bacteria are either housed in the multi-chambered stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
or in a large cecum
Cecum
The cecum or caecum is a pouch, connecting the ileum with the ascending colon of the large intestine. It is separated from the ileum by the ileocecal valve or Bauhin's valve, and is considered to be the beginning of the large intestine. It is also separated from the colon by the cecocolic...
. The size of an animal is also a factor in determining diet type. Since small mammals have a high ratio of heat-losing surface area to heat-generating volume, they tend to have high-energy requirements and a high metabolic rate
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
. Mammals that weigh less than about 18 oz (500 g) are mostly insectivorous because they cannot tolerate the slow, complex digestive process of a herbivore. Larger animals on the other hand generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (those that prey on larger vertebrates) or a slower digestive process (herbivores). Furthermore, mammals that weigh more than 18 oz (500 g) usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).
Specializations in herbivory include:
Granivory "seed eating", folivory "leaf eating", fruivory "fruit eating", nectivory "nectar eating", gumivory "gum eating", and mycophagy "fungus eating".
See also
- List of African mammals
- List of extinct mammals
- List of Indian mammals
- List of mammalogists
- List of mammal genera
- List of mammals
- Lists of mammals by region
- List of prehistoric mammals
- Mammal classificationMammal classificationMammalia is a class of animal within the Phylum Chordata. Mammal classification has been through several iterations since Carolus Linnaeus initially defined the class. Many earlier ideas have been completely abandoned by modern taxonomists, among these are the idea that bats are related to birds...
- Mammals discovered in the 2000sMammals discovered in the 2000sAlthough the mammals are well studied in comparison to other animal groups, a number of new species are still being discovered. This list includes extant mammal species discovered, formally named, or brought to public light in the year 2000 or later...
- Prehistoric mammalPrehistoric mammalPrehistoric mammals are groups of mammals that lived before humans developed writing. 164 million years ago, in the Jurassic period, Castorocauda lutrasimilis, a mammal-like animal weighing about 500 grams , had a full mammalian pelt, with guard hairs and under fur, webbed feet, and scales on the...
s
Further reading
- Bergsten, Johannes. February 2005. "A review of long-branch attraction". Cladistics 21:163–193. (pdf version)
- Khalaf-von Jaffa, Norman Ali Bassam Ali Taher (2006). Mammalia Palaestina: The Mammals of Palestine. Gazelle: The Palestinian Biological Bulletin. Number 55, July 2006. pp. 1–46.
- McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. ISBN 0-231-11013-8
- Nowak, Ronald M. 1999. Walker's Mammals of the World, 6th edition. Johns Hopkins University Press, 1936 pp. ISBN 0-8018-5789-9
- William J. Murphy, Eduardo Eizirik, Mark S. Springer et al., Resolution of the Early Placental Mammal Radiation Using Bayesian Phylogenetics,Science, Vol 294, Issue 5550, 2348–2351, 14 December 2001.
- Springer, Mark S., Michael J. Stanhope, Ole Madsen, and Wilfried W. de Jong. 2004. "Molecules consolidate the placental mammal tree". Trends in Ecology and Evolution, 19:430–438. (PDF version)
- Vaughan, Terry A., James M. Ryan, and Nicholas J. Capzaplewski. 2000. Mammalogy: Fourth Edition. Saunders College Publishing, 565 pp. ISBN 0-03-025034-X (Brooks Cole, 1999)
- Jan Ole Kriegs, Gennady Churakov, Martin Kiefmann, Ursula Jordan, Juergen Brosius, Juergen Schmitz. (2006) Retroposed Elements as Archives for the Evolutionary History of Placental Mammals. PLoS Biol 4(4): e91.
- David MacDonald, Sasha Norris. 2006. The Encyclopedia of Mammals, 3rd edition. Printed in China, 930 pp. ISBN 0-681-45659-0.
External links
- BBC Wildlife Finder – video clips from the BBC's natural history archive
- GlobalTwitcher.com – All species in the world with distribution maps and images
- Paleocene Mammals, a site covering the rise of the mammals, paleocene-mammals.de
- Evolution of Mammals, a brief introduction to early mammals, enchantedlearning.com
- Tree of Life poster – Shows mammals' evolutionary relation to other organisms, tellapallet.com
- The Evolution of Mesozoic Mammals, a Rough Sketch, an informal introduction, home.arcor.de
- Carnegie Museum of Natural History, some discoveries of early mammal fossils, carnegiemnh.org
- High-Resolution Images of various Mammalian Brains, brainmaps.org
- Mammal Species, collection of information sheets about various mammal species, learnanimals.com
- Summary of molecular support for Epitheria, biology.plosjournals.org
- Mikko's Phylogeny Archive, fmnh.helsinki.fi
- European Mammal Atlas EMMA from Societas Europaea Mammalogica, European-mammals.org
- Marine Mammals of the World—An overview of all marine mammals, including descriptions, multimedia and a key, eti.uva.nl
- Mammalogy.org The American Society of Mammalogists was established in 1919 for the purpose of promoting the study of mammals, and this website includes a mammal image library