Synapsids, the mammal line, split from reptiles

Amniotes divided early into two great lineages. One, the sauropsids, led to reptiles and birds. The other, the synapsids, led to mammals. Synapsids are named for a single opening in the skull behind each eye, where jaw muscles attached. Their oldest well-known members, such as Archaeothyris, from Nova Scotia, Canada, are about 306 million years old. Archaeothyris was a lizard-like animal about half a metre long with strong jaws and a pair of larger teeth.

Early synapsids are often called pelycosaurs or mammal-like reptiles, though they were never true reptiles. Some, like the carnivorous Dimetrodon and the plant-eating Edaphosaurus, grew a sail on the back. By the Middle Permian the group had produced the therapsids, with more upright limbs and stronger jaws, and later the cynodonts and mammals. Synapsids were the dominant land vertebrates for much of the Permian. Their later history includes the Great Dying, which nearly wiped them out, and a rebound that finally produced the first mammals.