Synapsida

Synapsida is a diverse group of tetrapodvertebrates 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 monophyleticsister 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 vertebrates in the Permian period (299 to 251 mya), rivalled only by some large pareiasaurianparareptiles such as Scutosaurus. They were the dominant land predators of the late Paleozoic and early Mesozoic, with eupelycosaurs such as Dimetrodon, Titanophoneus and Inostrancevia being the apex predators during the Permian, and theriodonts such as Moschorhinus during the Early Triassic. Synapsid population and diversity were severely reduced by the Capitanian mass extinction event and the Permian–Triassic extinction event, and only two groups of therapsids, the dicynodonts and eutheriodonts (consisting of therocephalians and cynodonts) are known to have survived into the Triassic. These therapsids rebounded as disaster taxa during the early Mesozoic, with the dicynodont Lystrosaurus making up as much as 95% of all land species at one time, but declined again after the Smithian–Spathian boundary event with their dominant niches largely taken over by the rise of archosaurian sauropsids, first by the pseudosuchians and then by the pterosaurs and dinosaurs. The cynodont group Probainognathia, which includes the group Mammaliaformes, were the only synapsids to survive beyond the Triassic, During the Jurassic, cynodonts lived mostly nocturnally to avoid competition with dinosaurs. After the Cretaceous–Paleogene extinction event wiped out all non-avian dinosaurs and pterosaurs, synapsids (as mammals) rose to dominance once again during the Cenozoic, with roughly 6,800 species living currently.