Therapsids were a group of synapsids, the tetrapods that sort of bridge the gap between ancestral reptiles and ancestral mammals. They evolved from the pelycosaurs, which included dimetrodons and their relatives. Dimetrodons had dominated the early Permian, but by later in the period therapsids were diversifying and finally became the dominant land vertebrates in late Permian time. Their skeletal structures were more mammal-like than reptilian, including the geometry of their legs and feet which were located more directly under the animal. This gave them a somewhat more upright posture than the sprawling, crocodilian stance of the reptiles. Their teeth were also evolving into the canines and incisors more typical of mammals.
|Permian therapsid drawn by Nobu Tamura (http://spinops.blogspot.com) used under GFDL|
One of the key features that separates modern mammals from reptiles is that mammals are warm-blooded. They don’t rely on the sun’s direct rays for warmth, but can maintain and manage their internal temperatures. Cynodonts – a name that means “dog teeth” – appeared in Late Permian time, and managed to survive the extinction at the end of the Permian. It appears that the cynodonts gave rise to true mammals during the Triassic Period. Cynodonts had a lot of mammalian characteristics, and many researchers think they were warm-blooded, so they might have been classified as mammals if we had one to look at. You’ll see them called half-mammal, half-reptile, missing links, and more, but I think the best way to think of the early cynodonts is as pre-mammals. No longer really reptiles, but perhaps lacking some of the traits we use to define mammals, such as hair and milk production. Or maybe they did have those things. I don’t think there are any Permian cynodont fossils with hair preserved, so we aren’t sure – but we’re definitely on the mammalian track.
Most of the known cynodonts from the Permian were small, the size of a rat or a cat.
—Richard I. Gibson
Permian therapsid drawn by Nobu Tamura (http://spinops.blogspot.com) used under GFDL