While the early therapsids had skulls very similar to those of their pelycosaurian ancestors, they differed in the post-cranial skeleton.
Legs and feet
Their legs are positioned more vertically beneath their bodies than are the sprawling legs of reptiles and pelycosaurs. The feet were more symmetrical, with the first and last toes short and the middle toes longer, an indication that the foot's axis was placed parallel to that of the animal, not sprawling out sideways. This would have given a more mammal-like gait than the lizard-like gait of the pelycosaurs.
Jaw and teeth
Therapsids' temporal fenestrae are greater than those of the pelycosaurs. The jaws of some therapsids are more complex and powerful, and the teeth are differentiated into frontal incisors for nipping, great lateral canines for puncturing and tearing, and molars for shearing and chopping food.
Like all land animals, the therapsids were seriously affected by the Permian–Triassic extinction event; the very successful gorgonopsians dying out altogether and the remaining groups - dicynodonts, therocephalians, and cynodonts - reduced to a handful of species each by the earliest Triassic. The dicynodonts, now represented by a single family of large stocky herbivores, the Kannemeyeridae, and the medium-sized cynodonts (including both carnivorous and herbivorous forms), flourished worldwide throughout the Early and Middle Triassic. They disappear from the fossil record across much of Pangea at the end of the Carnian (Late Triassic), although they continued for some time longer in the wet equatorial band and the south.
The therocephalians, relatives of the cynodonts, managed to survive the Permian-Triassic extinction and continued to diversify through the Early Triassic period. Approaching the end of the period, however, the therocephalians were in decline to eventual extinction, likely out competed by the rapidly diversifying Saurian lineage of diapsids, equipped with sophisticated respiratory systems better suited to the very hot, dry and oxygen-poor world of the End-Triassic.
Dicynodonts were long thought to have become extinct near the end of the Triassic, but there is evidence that they survived into the Cretaceous. Their fossils have been found in Gondwana. This is an example of Lazarus taxon. Other animals that were common in the Triassic also took refuge here, such as the temnospondyls.
A. K. Huttenlocker, and E. Rega, 2012. Chapter 4: The Paleobiology and Bone Microstructure of Pelycosauriangrade Synapsids. Pp. 90–119 in A. Chinsamy (ed. [clarification needed]) Forerunners of Mammals: Radiation, Histology, Biology. Indiana University Press.
Thulbord, Tony; Turner, Susan (2003). "The last dicynodont: an Australian Cretaceous relict". Proceedings of the Royal Society of London B270: 985–993. doi:10.1098/rspb.2002.2296.
Benton, M. J. (2004). Vertebrate Palaeontology, 3rd ed., Blackwell Science.
Carroll, R. L. (1988). Vertebrate Paleontology & Evolution. W. H. Freeman & Company, New York.
Kemp, T. S. (2005). The origin and evolution of mammals. Oxford University Press.
Romer, A. S. (1966). Vertebrate Paleontology. University of Chicago Press, 1933; 3rd ed.
Bennett, A. F., & Ruben, J. A. (1986). "The metabolic and thermoregulatory status of therapsids." In The ecology and biology of mammal-like reptiles. Smithsonian Institution Press, Washington, DC, 207-218.