Caseasauria
Eupelycosauria
- Edaphosauridae
- Sphenacodontia
- Sphenacodontidae
- Therapsida
Synapsids ("fused arch"), formerly known as "mammal-like reptiles", are a group of amniotes (the other being the sauropsids) that developed one hole in their skull (temporal fenestra) behind each eye, about 320 million years ago (Ma) during the late Carboniferous. The 3,500 species of living synapsids, known as mammals, are currently the dominant land animals, and include both aquatic (whales) and avian (bats) species, and the largest animal to ever exist (the blue whale).
The synapsids known as pelycosaurs and caseasaurs were the first successful group of amniotes, spreading and diversifying until they became the dominant large terrestrial animals in the late Carboniferous. They were sprawling, bulky, cold-blooded, had small brains, and were the largest land animals of their time, ranging up to 3 m (10 ft) in length. Many, like Dimetrodon, had large sails that may have helped raise their body temperature. They lasted through the Permian and became extinct during the Permian-Triassic extinction event, the mass extinction known, about 250 Ma.
The therapsids, a more advanced group of synapsids, appeared during the first half of the Permian, and went on to become the dominant large terrestrial animals during the latter half. Some groups survived the Permian extinction and went on to dominate the early Triassic as well, though the more lightly built diapsid amniotes, such as the thecodonts and archosaurs, started to become more common. The Triassic-Jurassic extinction event finally wiped out all large synapsids and left room for the diapsid archosaurs known as dinosaurs to spread, and they dominated the terrestrial ecosystem for the rest of the Mesozoic Era. The remaining Mesozoic synapsids were small, ranging from the size of a shrew, to the badger-like Repenomamus.
While marginalized, synapsids continued to evolve and developed sophisticated teeth. The first known mammal appeared during the Jurassic. There is no fossil evidence until much later because soft tissues are rarely preserved, but many traditional mammalian features, like bearing live young, milk-producing teats, fur, and warm-bloodedness, probably appeared in the cynodonts, the immediate ancestors of the mammals.
The hole in the synapsid head allows the attachment of larger lower jaw muscles. The synapsid lower jaw bone also gradually became just one large bone, with the smaller jaw bones migrating into the inner ear and allowing sophisticated hearing.
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