Megazostrodon
Encyclopedia
Megazostrodon is an extinct Mammaliaform
, widely accepted as being one of the first mammal
s, appearing in the fossil record approximately 200 million years ago. It did have some non-mammalian characteristics but they were minor enough to be able to say that this animal is likely to represent the final stage of the transition between cynodont
, or "mammal-like" reptiles and true mammals.
-like animal between 10 to 12 cm (3.9 to 4.7 in) long which probably ate insects and small lizards. It is thought that it was nocturnal as it had a much larger brain than its cynodont
relatives and the enlarged areas of its brain were found to be those that process sounds and smells. This was probably in order to avoid being in competition with the reptiles or becoming prey to the dinosaur
s.
s or 'mammal-like reptiles’) during the late Triassic
and early Jurassic
periods. However, recent classifications consider the megazostrodontids to be mammaliaforms just outside the Mammalia proper, while the triconodonts remain in the (crowngroup) Mammalia. One must keep in mind that the distinction between true mammals and mammaliaformes is purely a cladistic and phylogenetic one: mammaliaforms are mammals in every other way.
These early mammals developed many traits which were to make them well-suited for a very active lifestyle. They developed four types of teeth (as opposed to the uniform teeth of the reptiles), incisors, canines
, premolars and molars, which enabled them to chew and therefore process their food more thoroughly than their reptilian cousins. There is evidence that the inward-closing movement of the mandible suggests a shearing action to chew food. Their skeletons changed so that their limbs were more flexible (they became less laterally splayed, allowing for faster forward motion) and they developed a shorter ribcage and larger lungs to allow for faster respiration
. The structure of their jaw bones changed, the lower jaw becoming a single bone — the dentary (as opposed to the seven different bones found in reptilian lower jaws). The other bones which once made up the jaw moved to the middle ear to create a hearing system.
Probably the most important aspect of change in the evolution that led to these first mammals was that their direct ancestors (the cynodonts) had become warm-blooded
. This meant that they relied on the food they ate to help sustain their body temperature rather than depending on their surrounding environment, which enabled them to maintain higher activity levels during the day than reptiles could (as reptiles frequently have to perform temperature regulation activities such as sun basking and seeking shade) and even to become nocturnal — a major advantage in a world where most predators were active during the day.
s. Live birth and the placenta
would be later evolutions for mammals.
of southern Africa
by Ione Rudner (whose surname was given to the animal’s specific name) in 1966. It was, however, first described by A.W. Crompton and F.A. Jenkins Jr in 1968.
Its name means, literally, ‘Rudner’s large girdle tooth’ (from the Greek mega-large, zostros-girdle and odon-tooth — referring to the large external cingula, or ridges of the upper molars).
show Animal Armageddon
.
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...
, widely accepted as being one of the first 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, appearing in the fossil record approximately 200 million years ago. It did have some non-mammalian characteristics but they were minor enough to be able to say that this animal is likely to represent the final stage of the transition between 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...
, or "mammal-like" reptiles and true mammals.
Characteristics
Megazostrodon was a small, furry, shrewShrew
A shrew or shrew mouse is a small molelike mammal classified in the order Soricomorpha. True shrews are also not to be confused with West Indies shrews, treeshrews, otter shrews, or elephant shrews, which belong to different families or orders.Although its external appearance is generally that of...
-like animal between 10 to 12 cm (3.9 to 4.7 in) long which probably ate insects and small lizards. It is thought that it was nocturnal as it had a much larger brain than its 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...
relatives and the enlarged areas of its brain were found to be those that process sounds and smells. This was probably in order to avoid being in competition with the reptiles or becoming prey to the 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.
Evolution
Megazostrodon is the only well-known genus of the family Megazostrodontidae. The other members of this family that are currently known to science are Indozostrodon, Dinnetherium, Wareolestes and Brachyzostrodon. The megazostrodontids used to be classified as members of a group of mammals called the Triconodonts, which are thought to have evolved from a specific group of reptiles (the cynodontCynodont
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 or 'mammal-like reptiles’) during the late 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...
and 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...
periods. However, recent classifications consider the megazostrodontids to be mammaliaforms just outside the Mammalia proper, while the triconodonts remain in the (crowngroup) Mammalia. One must keep in mind that the distinction between true mammals and mammaliaformes is purely a cladistic and phylogenetic one: mammaliaforms are mammals in every other way.
These early mammals developed many traits which were to make them well-suited for a very active lifestyle. They developed four types of teeth (as opposed to the uniform teeth of the reptiles), incisors, canines
Canine tooth
In mammalian oral anatomy, the canine teeth, also called cuspids, dogteeth, fangs, or eye teeth, are relatively long, pointed teeth...
, premolars and molars, which enabled them to chew and therefore process their food more thoroughly than their reptilian cousins. There is evidence that the inward-closing movement of the mandible suggests a shearing action to chew food. Their skeletons changed so that their limbs were more flexible (they became less laterally splayed, allowing for faster forward motion) and they developed a shorter ribcage and larger lungs to allow for faster respiration
Respiratory system
The respiratory system is the anatomical system of an organism that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles...
. The structure of their jaw bones changed, the lower jaw becoming a single bone — the dentary (as opposed to the seven different bones found in reptilian lower jaws). The other bones which once made up the jaw moved to the middle ear to create a hearing system.
Probably the most important aspect of change in the evolution that led to these first mammals was that their direct ancestors (the cynodonts) had become warm-blooded
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...
. This meant that they relied on the food they ate to help sustain their body temperature rather than depending on their surrounding environment, which enabled them to maintain higher activity levels during the day than reptiles could (as reptiles frequently have to perform temperature regulation activities such as sun basking and seeking shade) and even to become nocturnal — a major advantage in a world where most predators were active during the day.
Reproduction
Despite all the mammalian traits that Megazostrodon acquired, it is thought that they still laid leathery eggs like their reptilian cousins, similar to the extant monotremeMonotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s. Live birth and the 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...
would be later evolutions for mammals.
Discovery
Megazostrodon rudnerae was first discovered in LesothoLesotho
Lesotho , officially the Kingdom of Lesotho, is a landlocked country and enclave, surrounded by the Republic of South Africa. It is just over in size with a population of approximately 2,067,000. Its capital and largest city is Maseru. Lesotho is a member of the Commonwealth of Nations. The name...
of southern Africa
Africa
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...
by Ione Rudner (whose surname was given to the animal’s specific name) in 1966. It was, however, first described by A.W. Crompton and F.A. Jenkins Jr in 1968.
Its name means, literally, ‘Rudner’s large girdle tooth’ (from the Greek mega-large, zostros-girdle and odon-tooth — referring to the large external cingula, or ridges of the upper molars).
Popular culture
Megazostrodon was one of the animals featured in the sixth episode of Animal PlanetAnimal Planet
Animal Planet is an American cable tv specialty channel that launched on October 1, 1996. It is distributed by Discovery Communications. A high-definition simulcast of the channel launched on September 1, 2007.-History:...
show Animal Armageddon
Animal Armageddon
Animal Armageddon is an American paleontology-based documentary television miniseries that originally aired from to on Animal Planet. All the prehistoric scenes are created 100% in Lightwave...
.