Synapsida

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 pelycosaurs or pelycosaur-grade synapsids. These paraphyletic terms have fallen out of favor and are only used informally (if at all) in modern literature, as it is now known that all extant reptiles are more closely related to each other and birds than to synapsids, so the word "reptile" has been re-defined to mean only members of Sauropsida or even just an under-clade thereof. In a cladistic sense, synapsids are in fact a monophyletic sister taxon of sauropsids, rather than a part of the sauropsid lineage. Modern alternatives sometimes used include stem mammals, proto-mammals, paramammals or pan-mammals. Most lineages of pelycosaur-grade synapsids were replaced by the more advanced therapsids, which evolved from sphenacodontian pelycosaurs, at the end of the Early Permian, a transition known as Olson's Gap/Extinction. The earliest synapsids appeared towards the end of the Carboniferous, around 315 to 307 million years ago. Synapsids were the largest terrestrial...