Megazostrodon
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Megazostrodon is an extinct Mammaliaform, widely accepted as being one of the first mammals to have appeared on Earth, first appearing in the fossil record approximately 200 million years ago.[1] 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.[2]
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[edit] Characteristics
Megazostrodon was a small, furry,[3] shrew-like animal between 10 and 12 cm long which probably ate insects and small lizards. It is thought that it was nocturnal as it had a much larger brain[4] than its cynodont relatives and the areas of its brain that created this variance in size were found to be those that process sounds and smells. This was probably in order to avoid being in competition with the reptiles and becoming prey to the dinosaurs.[5]
[edit] Evolution
Megazostrodon is one of a group of mammals called the Triconodonts which are thought to have evolved from a specific group of reptiles— the cynodonts, which are known as ‘mammal-like reptiles’—[6] at some point during the late Triassic and early Jurassic periods.
These early mammals developed many traits which were to make them well suited for a very active lifestyle. They developed four types of teeth[7] (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. Their skeletons changed so that their limbs were more flexible (they became less laterally splayed[8] allowing for faster forward motion) and they developed a shorter ribcage and larger lungs[7] 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[9] 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 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.
[edit] Reproduction
Megazostrodon suckled their young, which suggests a higher degree and longevity of parental care than is generally the case amongst reptiles (although there are thought to have been exceptions to this as there certainly are today; most reptiles are independent from birth and are often actually seen as prey items to their parents). This parental care would have provided higher nutrition levels and therefore speedy growth, and more opportunity for education of the youngsters-having to rely less heavily on hard-wired natural instinct. This method of raising young allows for a lower infant mortality rate and higher population increase.
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 monotremes. Live birth and the placenta would be later evolutions for mammals.
[edit] Discovery
Megazostrodon rudnerae was first discovered in Africa by Ione Rudner (who gave her surname to the animal’s specific name) in 1966.[10] It was, however, first described by A.W. Crompton and F.A. Jenkins Jr in 1968 in a paper entitled "Molar occlusion in late Triassic mammals".
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).
[edit] In popular culture
A Megazostrodon basks in When Dinosaurs Roamed America before being scared by a Coelophysis
[edit] Notes
- ^ Fur and Fangs: Mammal Origins. Palaeobiology and Biodiversity Research Group, University of Bristol.
- ^ Historical Biology. 1992, vol. 6 p.186
- ^ Its furriness is extrapolated from the fact that its skeletal morphology implies a warm-blooded animal with a high metabolism, and warm-blooded animals are covered with insulatiing features like feather, down, or hair in mammals.
- ^ Fur and Fangs: Mammal Origins. Palaeobiology and Biodiversity Research Group, University of Bristol.
- ^ Smithsonian Science Abstract
- ^ Mammology Sam Houston State University.
- ^ a b First Mammals Appear
- ^ Mammilian Characteristics. Sam Houston State University.
- ^ Fur and Fangs: Mammal Origins. Palaeobiology and Biodiversity Research Group, University of Bristol.
- ^ Dinosaur Mailing List
