Megazostrodon
Name:
Megazostrodon
(Large girdle tooth).
Phonetic: Meg-ah-zo-stro-don.
Named By: Crompton and Jennings - 1968.
Classification: Chordata, Synapsida,
Mammaliaformes, Morganucodonta, Megazostrodontidae.
Species: M. rudnerae (type).
Diet: Insectivore.
Size: 10 to 12 centimetres long.
Known locations: Southern Africa, Lesotho.
Time period: End of the Triassic to start of the
Jurassic.
Fossil representation: Almost complete specimen.
When
you look up the first true mammals in dinosaur books you almost
certainly going to be presented with a picture and brief description of
Megazostrodon. Modern analysis of Megazostrodon
has it in a position
that is almost fully mammal, yet not quite. True mammals begin with
the triconodont group, something that the Megazostrodontidae used to
belong to until analysis placed it just outside.
Despite
the revised classification, Megazostrodon's
adaptations are almost
totally mammalian, and it continues to be a strong transitional
form between the earlier therapsids and true mammals. The first key
area is that the mandible (lower jaw) became one single bone rather
than the seven seen in reptiles. However these bones were not lost
they actually got rearranged to form the middle ear, becoming the
mammalian hearing system. Megazostrodon also had
a covering of hair
over its body, something that was not necessarily new as suggested by
Thrinaxidon
and Cynognathus,
but still a clear mammalian
trait.
The
reduction of the ribs and enlargement of the lungs points towards a
development for a faster rate of respiration. This is of particular
importance when you realise that a faster rate of respiration is
required to support an endothermic (warm-blooded) metabolism.
Ectothermic (cold-blooded) metabolisms by comparison are generally
slower and do not require such a high intake of oxygen to burn food for
energy.
Connected
to the support of a warm blooded metabolism are the teeth which can be
arranged into clear incisor, canine, pre-molar and molar groups.
This allowed for efficient processing of food such as insects that
would require incisors and canines for capture and killing, and the
molars for crunching the hard exoskeleton. By chewing small prey like
insects Megazostrodon would have a high protein
diet that would be
quickly digested, a more efficient system that supplied the increased
demands of a faster metabolism.
There
are still two key reptilian features of Megazostrodon,
and one is the
way the legs sprawl outwards. Although the limbs are not as
pronounced as the earlier therapsids they still do not support the
body from directly underneath. This in mind however, the skeletal
joints do indicate a greater amount of movement was possible,
something that would increase the agility of Megazostrodon.
The
second feature is that Megazostrodon is thought to
have still laid eggs
like a reptile. However while it may sound strange for a mammal to
lay eggs, monotremes like the platypus and echidna are mammals
that do lay eggs today.
Despite
the egg laying Megazostrodon is thought to have
suckled its young,
something that would continue all the way down the line to mammals
today. By being warm-blooded Megazostrodon is
also thought to have
been nocturnal so as to avoid the cold-blooded predators that would
have been active during the day perhaps like the newly evolving
dinosaurs or other carnivorous archosaurs.
The
dating of the Megazostrodon specimen places it
around two hundred
million years ago, placing it just over into the Triassic of the
Jurassic/Triassic boundary. The immediate future of the new mammalian
creatures would be a small one that saw them scurrying around trying to
stay out of sight. Even a hundred and forty million years later
towards the end of the Cretaceous, the larger mammals like
Didelphodon
would only attain sizes of up to thirty centimetres.
Despite
these small sizes the mammals would quickly form an important part of
the ecosystem, keeping the number of insects down as they fed upon
them and in turn being food for other predators like the dinosaurs,
something which has be learned from the presence of Zhangheotherium
and Sinobataar
remains inside the dinosaur Sinosauropteryx.
Further reading
- Molar occlusion in Late Triassic mammals. - Biological Reviews
43:427-458. - A. W. Crompton & F. A. Jenkins - 1968.
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