Synapsida

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Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant reptiles and birds). Synapsids have a single lower temporal fenestra, an opening low in the skull roof behind each eye socket. These openings create a bony arch that allows for the attachment of larger jaw muscles and a more efficient bite. While previously argued to be distinctive to synapsids, single temporal fenestrae are also found in many primitive sauropsids. It has recently been argued that the last common ancestor of synapsids and sauropsids shared a single lower temporal fenestra. The basal amniotes (reptiliomorphs) from which synapsids evolved were historically simply called "reptiles". Therefore, non-mammalian synapsids were then described as mammal-like reptiles in classical systematics, and non-therapsid synapsids were also referred to as...

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