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Today, the 5,500 species of living synapsids, known as the mammals, include both aquatic and flying species, and the largest animal ever known to have existed (the blue whale). Humans are synapsids, as well. Most mammals are viviparous and give birth to live young rather than laying eggs with the exception being the monotremes.
Although some diapsids have lost either one hole (lizards), or both holes (snakes and turtles), or have a heavily restructured skull (modern birds), they are still classified as diapsids based on their ancestry. At least 17,084 species of diapsid animals are extant: 9,159 birds, [3] and 7,925 snakes, lizards, tuatara, turtles, and crocodiles. [4]
From top to bottom (A) a skull of an Anapsid, (B) a Synapsid (stem-mammal) skull, and (C) a Diapsid skull. [a] Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit (eye socket). These openings have historically been used to track the evolution and affinities of reptiles.
Synapsids had one opening on each side, while diapsids (a branch of Sauropsida) had two. An early, inefficient version of diaphragm may have evolved in synapsids. The earliest synapsids, or "proto-mammals," are the pelycosaurs. The pelycosaurs were the first animals to have temporal fenestrae. Pelycosaurs were not therapsids but their ancestors.
In anapsids, the ancestral condition, there are none; in synapsids (mammals and their extinct relatives) there is one; and in diapsids (including birds, crocodilians, squamates, and tuataras), there are two. Turtles have secondarily lost their fenestrae, and were traditionally classified as anapsids because of this.
Synapsids became the dominant type of animal, represented by the Pelycosaurs during the Early Permian and Therapsids during the Middle and Late Permian, and distinguished by the appearance and possession of mammal-like characteristics (hence the old term "mammal-like reptiles").
The synapsid lineage became distinct from the sauropsid lineage in the late Carboniferous period, between 320 and 315 million years ago. [2] The only living synapsids are mammals, [3] while the sauropsids gave rise to the dinosaurs, and today's reptiles and birds along with all the extinct amniotes more closely related to them than to mammals. [2]
From this reptile, three groups of amniotes would evolve: anapsids, diapsids, and synapsids. These broad groupings of amniotes are most easily differentiated by the presence and number of holes in the skull behind the eye socket. Those gaps, or holes, are called fenestrae, meaning "windows." The anapsids are the most primitive members of the group.