Morganucodonta
Encyclopedia
The morganucodonts are an extinct group of important animals that seem to represent the earliest of mammals. These were shrew-sized, likely warm-blooded and possibly possessed mammary gland
s.
The morganucodonts were most likely insectivorous and nocturnal, seeking prey while avoiding predators that were usually active by day. Such a trait would be inherited by their descendants to avoid the predatory dinosaurs.
They were equipped with an unusual structure , the "double-joint", which meant that the jaw articulation would be made up of the dentary-squamosal
joint as well as a quadrate-articular one. Both the articular
and quadrate would eventually become the malleus
and incus
. There is a trough at the back of the jaw that houses postdentary bones, such bones are absent today in mammals (all living mammal
s today have a jaw
that is composed of a single bone, one of the defining features of Mammalia).
Unlike Sinoconodon
and the therapsids, the teeth are diphyodont (meaning that they possessed two sets of teeth as in all living mammals) and not polyphydont (meaning that the teeth are constantly replaced, as in reptiles). Furthermore, the primitive postcanine teeth found in more archaic proto-mammals have been replaced by true molars
and premolar
s. The teeth are structured in such a way that a one-to-one occlusion
and wear facets are present.
The septomaxilla, a primitive feature also found in Sinoconodon, is present as well as a fully ossified orbitosphenoid. The anterior lamina is enlarged. The cranial moiety of the squamosal
is a narrow bone that is superficially placed to the petrosal and parietal
. Unlike its predecessors, the morganucodonts have a larger cerebral capacity and a longer cochlea
.
The atlas elements are unfused, there is a suture between the dens and axis
. The cervical rib
s are not fused to the centra
. The coracoid and procoracoid, which are absent in therian
s, are present. The head of the humerus
spherical as in mammals, but the spiral ulnar condyle is cynodont
-like. In the pelvic girdle, the pubis
, ilium
and ischium are unfused.
, they were equipped with a double jaw-joint instead (see above). Morganucodonts may be tagged as "prototherians."
, Western Europe
, Arizona
and China
.
The Illustrated Encyclopedia of the Prehistoric World, consultant editor Dr. Douglas Palmer. Page 342.
Zofia Kielan-Jaworowska, Richard L. Cifelli, and Zhe-Xi Luo, Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure (New York: Columbia University Press, 2004), 168-183.
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.
The morganucodonts were most likely insectivorous and nocturnal, seeking prey while avoiding predators that were usually active by day. Such a trait would be inherited by their descendants to avoid the predatory dinosaurs.
Anatomy
- JawJawThe jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...
They were equipped with an unusual structure , the "double-joint", which meant that the jaw articulation would be made up of the dentary-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,...
joint as well as a quadrate-articular one. Both 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...
and quadrate would eventually become the 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...
and 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....
. There is a trough at the back of the jaw that houses postdentary bones, such bones are absent today in mammals (all living mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s today have a jaw
Jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...
that is composed of a single bone, one of the defining features of Mammalia).
- Dentition
Unlike Sinoconodon
Sinoconodon
Sinoconodon rigneyi is an ancient proto-mammal that appears in the fossil record in the late Triassic period, about 208 million years ago. Although the animal seems more related to Morganucodon than anything else, it differed substantially from other Mammaliaformes in its dental and growth habits...
and the therapsids, the teeth are diphyodont (meaning that they possessed two sets of teeth as in all living mammals) and not polyphydont (meaning that the teeth are constantly replaced, as in reptiles). Furthermore, the primitive postcanine teeth found in more archaic proto-mammals have been replaced by true molars
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
and premolar
Premolar
The premolar teeth or bicuspids are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant, making eight premolars total in the mouth. They have at least two cusps. Premolars can be considered as a 'transitional tooth' during chewing, or...
s. The teeth are structured in such a way that a one-to-one occlusion
Occlusion (dentistry)
Occlusion, in a dental context, means simply the contact between teeth. More technically, it is the relationship between the maxillary and mandibular teeth when they approach each other, as occurs during chewing or at rest....
and wear facets are present.
- SkullSkullThe skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
The septomaxilla, a primitive feature also found in Sinoconodon, is present as well as a fully ossified orbitosphenoid. The anterior lamina is enlarged. The cranial moiety of 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,...
is a narrow bone that is superficially placed to the petrosal and parietal
Parietal
Parietal may refer to:*Parietal placentation*Parietal lobe of the brain*Parietal bone of the skull*Parietal scales of a snake lie in the general region of the parietal bone*Parietal cell in the stomach*Parietal pleura...
. Unlike its predecessors, the morganucodonts have a larger cerebral capacity and a longer cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
.
- Post-cranial skeleton
The atlas elements are unfused, there is a suture between the dens and axis
Axis (anatomy)
In anatomy, the second cervical vertebra of the spine is named the axis or epistropheus.It forms the pivot upon which the first cervical vertebra , which carries the head, rotates....
. The cervical rib
Cervical rib
A cervical rib is a supernumerary rib which arises from the seventh cervical vertebra. It is a congenital abnormality located above the normal first rib. A cervical rib is present in only about 1 in 500 of people; in even rarer cases, an individual may have two cervical ribs...
s are not fused to the centra
Centra
Centra is a convenience store chain in Ireland.The chain is run by Musgrave, the Irish food wholesaler, however the stores are all owned by individual franchisees. The chain has three different formats available to franchisees — smaller Quick Stop outlets, mid-sized Foodmarkets, and larger...
. The coracoid and procoracoid, which are absent in therian
Therian
Therian in the English language has two distinct definitions:* In taxonomy, the term refers to a member of the Mammalia subclass Theria, consisting of marsupial and placental mammals....
s, are present. The head of the humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
spherical as in mammals, but the spiral ulnar condyle is 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...
-like. In the pelvic girdle, the pubis
Pubis
Pubis may refer to:* Pubis * Mons pubis, a padding of fat that protects the pubis bone...
, ilium
Ilium
-Places:* Ilion or, Latinized, Ilium, another name for the legendary city of Troy, hence the title of Homer's Iliad*Ilium , an ancient city in Epirus...
and ischium are unfused.
True mammals?
Scientists still debate if the morganucodonts should be classified as true mammals or classified as a clade outside of mammalia. An argument that is often used to classify them as non-mammals is the fact that they did not possess the three middle ear bonesOssicles
The ossicles are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth . The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss...
, they were equipped with a double jaw-joint instead (see above). Morganucodonts may be tagged as "prototherians."
Geographical Distribution
Their remains have been found in southern AfricaAfrica
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, Western Europe
Western Europe
Western Europe is a loose term for the collection of countries in the western most region of the European continents, though this definition is context-dependent and carries cultural and political connotations. One definition describes Western Europe as a geographic entity—the region lying in the...
, Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
and China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
.
Classification
- Order Morganucodonta
- Family Morganucodontidae
- Genus MorganucodonMorganucodonMorganucodon is an early mammalian genus which lived during the Late Triassic. It first appeared about 205 million years ago. This has also been identified with Eozostrodon. Unlike many other early mammals, Morganucodon is well represented by abundant and well preserved, though in the vast...
- Genus EozostrodonEozostrodonEozostrodon was one of the earliest mammals. It lived during the late Triassic and the early Jurassic, about 210 million years ago. Eozostrodon was one of the largest early mammals, measuring more than a meter long....
- genus ?Gondwanadon
- Genus Helvetiodon
- Genus ErythrotheriumErythrotheriumErythrotherium is an extinct genus of basal mammal from the Lower Jurassic. It was related to Morganucodon. Only one species is recorded, Erythrotherium parringtoni, from Red Beds, Stromberg Group, Mafeteng and Upper Elliot Formation and Clarens Formation, from Lesotho and South...
- Genus Wareolestes
- Genus Morganucodon
- Family Megazostrodontidae
- Genus MegazostrodonMegazostrodonMegazostrodon is an extinct Mammaliaform, widely accepted as being one of the first mammals, appearing in the fossil record approximately 200 million years ago...
- Genus Brachyzostrodon
- Genus Dinnetherium
- Genus ?Indozostrodon
- Genus Megazostrodon
- Family Morganucodontidae
Sources and Further Reading
The Fossil Book: A Record of Prehistoric Life, authored by Patricia Vickers Rich, Thomas Hewitt Rich, Mildred Adams Fenton and Carroll Lane Fenton. Page 519.The Illustrated Encyclopedia of the Prehistoric World, consultant editor Dr. Douglas Palmer. Page 342.
Zofia Kielan-Jaworowska, Richard L. Cifelli, and Zhe-Xi Luo, Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure (New York: Columbia University Press, 2004), 168-183.
See also
- SinoconodonSinoconodonSinoconodon rigneyi is an ancient proto-mammal that appears in the fossil record in the late Triassic period, about 208 million years ago. Although the animal seems more related to Morganucodon than anything else, it differed substantially from other Mammaliaformes in its dental and growth habits...
- Adelobasileus
- HadrocodiumHadrocodiumHadrocodium wui is an extinct basal mammal species that lived during the Lower Jurassic in what is now the Yunnan province of China...
- Evolution of mammalsEvolution of mammals__FORCETOC__The evolution of mammals within the synapsid lineage was a gradual process that took approximately 70 million years, beginning in the mid-Permian. By the mid-Triassic, there were many species that looked like mammals, and the first true mammals appeared in the early Jurassic...
- Evolution of mammalian auditory ossiclesEvolution of mammalian auditory ossiclesThe evolution of mammalian auditory ossicles is one of the most well-documented and important evolutionary events, demonstrating both numerous transitional forms as well as an excellent example of exaptation, the re-purposing of existing structures during evolution.In reptiles, the eardrum is...