Therapsid

Therapsida (not to be confused with theropsida, which is its superclade) is a group of synapsids that includes mammals and their ancestors. Many of the traits today seen as unique to mammals had their origin within early therapsids, including having their four limbs extend vertically beneath the body, as opposed to the sprawling posture of reptiles. The earliest fossil attributed to Therapsida is Tetraceratops insignis from the Lower Permian. Therapsids evolved from 'pelycosaurs', specifically within the Sphenacodontia, more than 275 million years ago. They replaced the 'pelycosaurs' as the dominant large land animals in the Middle Permian and were largely 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 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. Compared to their pelycosaurian ancestors, early therapsids had very similar skulls but very different post-cranial morphology. Therapsid legs were positioned more vertically beneath their bodies than were the sprawling legs of reptiles and pelycosaurs. Also compared to these groups, the feet were more symmetrical, with the first and last toes short and the middle toes long, an indication that the foot's axis was placed parallel to that of the animal, not sprawling out sideways. This orientation would have given a more mammal-like gait than the lizard-like gait of the pelycosaurs. Therapsids' temporal fenestrae were larger than those of the pelycosaurs. The jaws of some therapsids were more complex and powerful, and the teeth were differentiated into frontal incisors for nipping, great lateral canines for puncturing and tearing, and molars for shearing and chopping food. Several characteristics in therapsids have been noted as being consistent with the development of endothermy: the presence of turbinates, erect limbs, highly vascularized bones, limb and tail proportions conducive to the preservation of body heat, and the absence of growth rings in bones. Therefore, like modern mammals, non-mammalian therapsids were most likely warm-blooded. Recent studies on Permian coprolites showcase that hair was present in at least some therapsids. Hair is by any means present in the docodont Castorocauda and several contemporary haramiyidans, and whiskers are inferred from therocephalians and cynodonts.