The therapsids were much more upright than their pelycosaur ancestors, their limbs being closer in to their bodies, decreasing the lateral undulation of the body and the characteristic waddle of earlier groups. This increased their freedom of movement, allowing for active foraging and for an increase in the ability of the respiratory and cardiovascular systems to sustain aerobic activity. These advances led to high metabolic rates and ultimately the endothermy characteristic of modern mammals.
The Biarmosuchia were the most basal of the therapsids. Tetraceratops from the Mid Permian of Texas, USA is the oldest known therapsid, and fits within this group. There is a significant reduction in teeth from the sphenacontid ancestors, and several alterations in the location of bones within the skull, but most evidence shows them as quite small predators, with intermediate features between pelycosaurs and later therapsid types.
The Suborder Dinocephalia contains about 40 genera of both carnivores and herbivores. Moschops, a massive herbivore found in South Africa was a dinocephalian, which reached lengths of up to 5 metres. The feet are surprisingly small, with the hindlimbs being under the body in the posture seen in modern mammals, but with the forelimbs still only capable of adopting a sprawling position. It also has a tiny head in comparison with the size of its body, with the roof of the head being up to 10cm thick, which some have suggested as an adaptation to headbutting behaviours.
Dicynodon skull, note the massive triangular synapsid arch, and solitary canine in the jaw to allow shearing bites through tough plant material.Reproduced with kind permission from Benton and Harper, 1997
The dominant herbivores in the Late Permian were the dicynodonts, making up 80-90% of species diversity in some communities. They were wide ranging in size, the largest being about three metres long, and bore a certain resemblance to the modern pig, only with a short triangular stump of a tail rather than the curly one we know today. Dicynodonts had short snouts, with a highly motile jaw, to slice and tear at the tough plant material that was its diet. Most dicynodonts were wiped out in the end-Permian mass extinction, but several lines survived, and radiated in the Triassic.
The skull of a gorgonopsian with synapsid arch, and massively enlarged canine teeth.Reproduced with kind permission from Benton and Harper, 1997
During the reign of the dicynodonts the corresponding dominant carnivores were the gorgonopsians. All examples of this suborder are around a metre in length with large fangs that did not close together, but sheared past each other, slicing through the flesh of their prey. This was essential for feeding from dicynodonts and dinocephalians in order pierce their thick skins.
The suborder Cynodontia is perhaps the most interesting of all the synapsid groups because of its legacy to the world: these were the ancestors of all living mammals. The group arose in the Mid Permian, with an increase in jaw muscles, and skull modifications to accommodate the extra movement needed for these muscles to contract. There was also an increase in dentition, re-filling the jaws with many teeth, fewer incisors and more cheek teeth. Further jaw modifications allowed later cynodonts to actually chew their food, allowing more differentiation of the teeth, and a greater specialisation of diet. Although most groups were carnivorous, three families, diademodontids, traversodontids and tritylodontids became herbivorous secondarily, being the first tetrapods to wear down their teeth into upper and lower profiles which matched exactly, allowing for extremely efficient processing of food.
It was in the later cynodonts that the mammalian
posture was finally widely adopted for all limbs, with locking
of joints to support the weight on the long axes of the bones,
and not rely upon muscle tension as fully. This also allowed full
freedom of movement, and a greatly improved efficiency of locomotion.
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