Reptiles - Dicynodonts and Cynodonts

The sail-backed reptiles were extinct by the middle of the Permian, and were replaced by two main groups of their descendants. The most abundant of these were the plant-eating dicynodonts. Lystrosaurus is a well-known dicynodont. Its remains are found in lower Triassic rocks in South Africa, India, China and Antarctica, again suggesting that Africa, Asia and Antarctica were once joined and have since separated.

Lystrosaurus was about one metre long and had tusk-like teeth in
the upper jaw and nostrils high on its snout. It probably lived
like a present-day hippopotamus, swimming and wading in
the swamps and breathing through its raised nostrils.

The mammal-like reptiles

The advanced cynodonts such as cynognathus were the ancestors of the mammals (warm-blooded animals covered in hair or fur), including humans. The cynodonts developed many mammalian features and they are often called the mammal-like reptiles. Their posture changed from the reptile pattern with the legs sprawled out at the sides, where the body has to be dragged over the ground, to a position under the body. This greatly improved the ability of these animals to run. The roof of the mouth also slowly changed and enabled the animal to continue breathing while it was eating. This again made it possible for the cynodonts to be more active. Other changes occurred in the structure of the limbs and skull, and particularly in the way the jaws moved.

Cynodonts may have had hair

It is possible that at least some of the cynodonts were covered with a dense coat of hair, and may therefore have been warm-blooded. Thrinaxodon was a small cynodont, well along the road to being a mammal. A fossilized adult was found in South Africa near a young individual. Some scientists think this may have been a mother protecting or suckling her young.

Something went wrong for the cynodonts. The carnivorous cynodonts flourished in the early and middle Triassic, but the dicynodonts declined. It seemed that the cynodonts and their descendants, the mammals, were going to become the most common and important animals on the land. But something must have gone wrong, and only a small trickle of the cynodonts and their descendants survived to carry on into the next period. It is true that one of these 'trickles', the mammals, did eventually go on to dominate the land. But it was about 120 million years, a long time even in geological terms, before that happened. First the 'Age of Dinosaurs' had to pass before the cynodonts could develop into lions, bears, mice, humans, and the other familiar present-day mammals.

Cynognathus was a carnivore as large as a wolf with
dagger-like canine teeth.