CYNODONT

(suborder or infraorder Cynodontia), mammallike reptiles of the order Therapsida (see therapsid) that existed from the Late Permian to the Early Jurassic Epoch (258 to 187 million years ago). Cynodont fossils have been found in China, South Africa, South America, and North America. (Examples in North America were not reported until 1989, from sites in Virginia, U.S.) The cynodont skull has many features anticipating that of a mammal. Notably, it has a secondary palate, as in a mammal, allowing it to simultaneously chew food and breathe, making for quicker digestion. (Most reptiles do not chew food but swallow it in large pieces, digesting slowly.) Thus cynodonts probably had a metabolic rate and large food requirement similar to mammals. They also had differentiated teeth, jaw bones, and jaw muscles suitable for chewing. Though the brain remained small, the braincase had an almost mammalian appearance, presaging the enlargement of the brain in mammalian descendants. Otherwise, the skeleton remained largely reptilian (the front and back limbs were appended on the sides, not underneath the body), signifying a sprawling posture and gait. The suborder Cynodontia contains, according to some classifications, five familiesProcynosuchidae, Galesauridae, Tritylodontidae, Chiniquodontidae, and Trithelodontidae. The first mammals probably derived from small carnivorous chiniquodontids or trithelodonts sometime in the Middle Triassic Epoch (240 to 230 million years ago).