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An advanced type of synapsid reptile that evolved in the Permian Period. Therapsids further differentiated their dentition into nipping, biting, and crushing teeth, and (unlike diapsids) had forelimbs that were more greatly developed than hindlimbs. Therapsids include the so-called mammallike reptiles of the Permian and Triassic Periods, as well as mammals. Compare anapsid, diapsid, synapsid.
any member of a major order (Therapsida) of reptiles of Permian and Triassic time (from 299 million to 200 million years ago). Therapsids were the stock that gave rise to mammals. As early as the preceding Carboniferous Period (from 359 million to 299 million years ago), there appeared a distinct evolutionary line, beginning with the archaic mammal ancestors, order Pelycosauria, and leading toward mammals. From one pelycosaur family sprang the therapsids. Therapsids include mammals and other cynodonts; they form a subgroup of the Synopsida, one of the major branches of amniotes. Therapsids first appear in the Permian Period, during which they flourished and evolved into a number of mammal forms