Therapsids, ancestors of mammals

Therapsids were a group of synapsids, the branch of amniotes that split from the reptile line early and eventually led to mammals. Often called mammal-like reptiles, they became common in the Middle Permian, about 270 million years ago, and ruled the land for tens of millions of years. They included the saber-toothed gorgonopsians, the beaked, plant-eating dicynodonts and the more lightly built cynodonts. Unlike earlier synapsids, therapsids held their legs more directly beneath the body and had teeth differentiated into incisors, canines and cheek teeth, a pattern we still have.

Nearly all therapsids were lost in the Great Dying about 252 million years ago, when roughly 70 percent of land vertebrate species vanished. A few lineages survived, among them the dicynodont Lystrosaurus, which spread across Pangaea in the aftermath. Another survivor was the cynodonts, small and active animals, some of which may already have had insulating fur, though debated. Over millions of years their jaws changed: bones at the back of the jaw shrank and moved into the ear, while a single bone, the dentary, formed the lower jaw. Mammals emerged from the cynodonts in the Late Triassic.