Synapsida

Synapsida is one of the two main branches of Amniota, the other being Sauropsida (which includes modern reptiles and birds). This group contains all mammals and their extinct relatives. A key feature is a single opening low in the skull behind each eye, called the lower temporal fenestra, which created a bony arch for larger jaw muscles and a stronger bite. However, this opening is not unique to synapsids—it also appears in many early sauropsids, and recent evidence suggests the last common ancestor of both groups already had it.

The earliest synapsids emerged near the end of the Carboniferous, roughly 315 to 307 million years ago. During the Permian (299 to 251 million years ago), they were the largest land animals, rivaled only by some big pareiasaurian parareptiles like *Scutosaurus*. They dominated as predators in the late Paleozoic and early Mesozoic, with forms like *Dimetrodon*, *Titanophoneus*, and *Inostrancevia* as apex predators in the Permian, and theriodonts like *Moschorhinus* in the Early Triassic. Their numbers and diversity crashed during the Capitanian mass extinction and the Permian–Triassic extinction event. Only two therapsid groups—dicynodonts and eutheriodonts (which include therocephalians and cynodonts)—survived into the Triassic. These rebounded as disaster taxa, with the dicynodont *Lystrosaurus* making up as much as 95% of all land species at one point, but they declined again after the Smithian–Spathian boundary event as archosaurian sauropsids (first pseudosuchians, then pterosaurs and dinosaurs) took over their niches. The only synapsids to persist beyond the Triassic were the cynodont group Probainognathia, which includes Mammaliaformes. During the Jurassic, cynodonts lived mostly nocturnally to avoid dinosaur competition. After the Cretaceous–Paleogene extinction event wiped out non-avian dinosaurs and pterosaurs, synapsids (as mammals) rose to dominance again in the Cenozoic, with about 6,800 living species today.

Historically, non-mammalian synapsids were called "mammal-like reptiles" or "pelycosaurs," and early synapsids were considered a reptile subclass. These terms are now outdated because reptiles are defined as part of Sauropsida, the sister group to Synapsida. In cladistic terms, synapsids are a monophyletic sister taxon to sauropsids, not a reptile lineage. Modern alternatives include stem mammals, proto-mammals, paramammals, or pan-mammals. Most pelycosaur-grade synapsids were replaced by more advanced therapsids (which evolved from sphenacodontian pelycosaurs) at the end of the Early Permian, a shift called Olson’s Gap or Olson’s Extinction.

For convenience, synapsids are split into two groups: therapsids (the advanced branch from which mammals evolved) and stem mammals (formerly pelycosaurs), which include six more primitive families. Stem mammals were lizard-like with sprawling gaits and possibly horny scutes, while therapsids tended toward more erect postures and possibly hair in some forms. In traditional taxonomy, these represent two grades, but modern classification treats Synapsida as a single clade.