Synapsid

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Synapsids

Image:Dimetrodon.jpg
Dimetrodon grandis skeleton at the </br> National Museum of Natural History

Scientific classification

Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Superclass:	Tetrapoda
(unranked)	Amniota
Class:	Synapsida Template:Taxobox authority

Clades

Caseasauria
Pelycosauria

Synapsids ("fused arch"), traditionally known as "mammal-like reptiles", were a class 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 class Synapsida is intentionally paraphyletic, constituting a grade of animals from the earliest split with sauropsids to the arbitrary division with its daughter class, Mammalia. These 4,500 species of living synapsids are currently the dominant land animals, and include both aquatic (whales) and flying (bats) species, and the largest animal ever known to have existed (the blue whale).

Archaeothyris was the very first known synapsid and it belonged to a group called pelycosaur, it lived in Pennsylvanian time of the Carboniferous period.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 mass extinction about 250 Ma, the largest extinction in Earth's history, which may have been related to the Siberian Traps volcanic event.

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.

From synapsids came the first mammal precursors, therapsids, and more specifically the eucynodonts, 220 million years ago (mya) during the Triassic period.

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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