Aspects of this topic are discussed in the following places at Britannica.
The suborder Cynodontia contains, according to some classifications, five families—Procynosuchidae, 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).
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