What we are, the Synapsids appeared around the end of the Carboniferous to the beginning of the Permian as the Pelycosaurs A.K.A the "Sailbacks", the best known include Dimetrodon and Edaphosaurus, the Pelycosaurs then became the Therapsids, such as the Gorgonopsids, Phthinosuchids, Biarmosuchids ect etc, these families are Eutheriodonts (except the oldest family, the Gorgonopsids which are Theriodonts), Humans as well as all mammals are Eutheriodonts as humans and mammals are descended from the Cynodonts, which are descended from the Therocephalians. After the Cynodonts came the mammals, the sole surviving class of the Synapsid legacy (reptiles are the Diapsid/Anapsids), Early in Synapsid history, we began splitting off from our reptilian brothers (before then we had all been Amniotes), the Synapsids and the Reptiles (Anapsids/Diapsids) began competing for survival, all throughout the Permian the Synapsids were the top predators until the the Permo-Triassic extinction, all the large synapsids (the predators like Gorgonops) were made extinct, only the small burrowers and the herbivores survived, the reptiles "took the crown" off the Synapsids and became the top predators, these were the Dinosaurs, they became as successful as the Synapsids before them.
After the K/T Extinction Event, the Synapsids regained the crown for being the dominant land-predators, like our forerunners about 200 million years before.
a Synapsid is one of few organisms to have been through both blood temperatures (e.g the early Synapsids (like Dimetrodon) were probably cold-blooded, probably why they had a sail, to heat them up quicker, their descendants, the later Synapsids (like the Mammals) are warm-blooded)
by Iameverywhereyetno-whereatall August 26, 2009

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