Synapsida

Pelycosauria

Synapsid. Synapsids were the largest terrestrial vertebrates in the Permian period, 299 to 251 million years ago.

As with almost all groups then extant, their numbers and variety were severely reduced by the Permian–Triassic extinction. Though some species survived into the Triassic period, archosaurs became the largest and most numerous land vertebrates in the course of this period. Few of the nonmammalian synapsids outlasted the Triassic, although survivors persisted into the Cretaceous. However, as a phylogenetic unit, they included the mammals as descendants, and in this sense synapsids are still very much a living group of vertebrates. After the Cretaceous–Paleogene extinction event, the synapsids (in the form of mammals) again became the largest land animals.