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THE MAMMAL LIKE
REPTILES
The word Dicynodont means "two dog teeth". The
dicynodonts were the most successful of the therapsids in terms of phylogenetic
longevity, numbers of individuals, and the extent of distribution. In fact they
were the most successful and wide-ranging group of plant-eating animals of the
time. They first appeared in the mid Permian and by the late Permian they were
among the commonest of all tetrapods. They were greatly decimated by the
increasingly harsh conditions of the terminal Permian, and only two lineages survived into the Early Triassic.
Although fossils in
Australia
suggest they may have survived as late as the Cretaceaous.
Dicynodont
Permian Russia
Lystrosaurus L. Triassic,
Karoo, South Africa
The body was short and broad, and was supported in the typical therapsid fashion
by strong thick limbs. Their heads had two small downward pointing tusks and a
blunt beak like a tortoise. Most of the Dicynodont skeleton is reptile like but
the presence of a single hole in the side of the skull behind the eye socket
that is typical of mammals. The way that their back legs are located directly
beneath their body instead of splaying out sideways is also more like a mammal
than a reptile. Furthermore, in some species only the males have tusks, this
type of sexual dimorphism is also typical of mammals. Dicyodonts probably lived
near rivers and lakes and were herbivores grazing the coarse vegetation on the
banks (tearing it off with their beaks and grinding it between their jaws that
could slide backwards and forwards.)
The advanced development of their skulls and jaws was the main factor in
the success of these animals. The temporal openings at the back of the skull
that are shared by all synapsids were greatly enlarged, so that the remaining
bone formed long arches. These large openings supported very powerful jaw
muscles. The dicynodonts skull is striking because of its light
open construction and the presence of long, bony bars rather than broad
plate-like areas behind the eye.
The dentition of these therapsids was unique. The front of the skull and
the lower jaw were narrow and beak-like. Apart from a pair of large, upper tusks
in some species, teeth were reduced to tiny remnants or were completely absent.
Notes on restoration of Russian skull. 1
tusk, minor area of the premax a little on the thiner parts of
the jugals, and there are no lacrimals.
FURTHER
READING
Gillian King: The Dicynodonts:
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Dicynodont
Permian Russia
Lystrosaurus L. Triassic,
Karoo, South Africa