Therapsida is a group of synapsids, and includes mammals and their ancestors.
Many of the traits today seen as unique to mammals had their origin within early therapsids, including an erect posture. The earliest fossil attributed to Therapsida is Tetraceratops insignis from the Lower Permian. Therapsids evolved from pelycosaurs (specifically sphenacodonts) 275 MYA. They replaced the pelycosaurs as the dominant large land animals in the Middle Permian and were replaced, in turn, by the archosauromorphs in the Triassic, although one group of therapsids, the kannemeyeriiforms, remained diverse in the Late Triassic. The therapsids included the cynodonts, the group that gave rise to mammals in the Late Triassic around 225 million years ago. Of the non-mammalian therapsids, only cynodonts and dicynodonts survived the Triassic–Jurassic extinction event. The last of the non-mammalian therapsids, the tritylodontid cynodonts, became extinct in the Early Cretaceous, approximately 100 million years ago.
In Walking with... seriesEdit
The last representatives of the Therapsida, the cynodonts (Thrinaxodon) and the dicynodonts (Placerias) were featured in this episode. Placerias was shown flourishing at the beginning of the episode, but dying out by the end of it, replaced by early plant-eating dinosaurs such as Plateosaurus. Thrinaxodon, on the other hand, survived throughout the episode, regardless of drought and carnivorous dinosaurs (Coelophysis), eventually giving rise to true mammals (as shown by the episodes 5 and 6 of the series).
Gorgonopsid, one of the biggest Therapsids ever. It was a dominant predator through the late Permian period but died-out in the mass extinction that ended the Paleozoic Era. By contrast, the smaller dicynodonts, the Diictodon, survived both the predators (like the gorgonopsids) and the mass extinction and even gave rise to bigger species, the Lystrosaurus, which was one of the most wide-spread herbivores of the early Triassic; both primitive archosaurs like Proterosuchus and other Therapsids, like the Therocephalians, hunted it. However, this episode also showed Euparkeria, an up-and-coming rival to the Lystrosaurus, which would eventually evolve into dinosaurs and replace the dicynodonts as herbivores.