Synapsids (Greek:
‘fused arch’; synonymous with theropsids (Greek,
‘beast-face’) –not to be confused with therapsids (Greek:
‘beast-arch’), which are a subordinate group to synapsids– are a group of
animals that includes mammals and every animal more closely related to mammals
than to the other members, reptiles and birds, included in
the amniotes clade. They are easily separated from other amniotes by having
a temporal fenestra, an opening low in the skull roof behind
each eye, leaving a bony arch beneath each; this accounts for their
name.Primitive synapsids are usually called pelycosaurs or
pelycosaur-grade synapsids. This informal term consists of all
synapsids which are not therapsids, a monophyletic more advanced mammal-like group.
The non-mammalian synapsids are described as mammal-like reptiles in
classical systematics; they can also be called stem mammals or proto-mammals.Synapsids
evolved from basal amniotes and are one of the two major groups of the later
amniotes, the other being the sauropsids,
a group that includes modern reptiles and birds. The distinctive temporal
fenestra developed in the ancestral synapsid about 312 million years ago,
during the Late Carboniferous period.

  Synapsids were the largest terrestrial
vertebrates in the Permian period,
299 to 251 million years ago, although some of the larger pareiasaurs at
the end of the Permian could match them in size. As with other groups then
extant, their numbers and variety were severely reduced by the Permian–Triassic extinction. By the time of the extinction at
the end of the Permian, all the older forms of synapsids (known as pelycosaurs)
were already gone, having been replaced by the more advanced therapsids.
Although the dicynodonts and eutheriodonts,
the latter consisting of the Eutherocephalia (Therocephalia)
and Epicynodontia (Cynodontia),
continued into the Triassic period as the only known surviving
therapsids, archosaurs became the largest and most numerous land
vertebrates in the course of this period. The Lisowicia bojani,
a discovery first reported in 2008, was the size of an elephant. The cynodont
group Probainognathia, which includes Mammaliaformes,
were the only synapsids who outlasted the Triassic. After the Cretaceous–Paleogene extinction event, the synapsids (in the
form of mammals) again became the largest land animals as well as becoming the
largest marine animals.