Amniotes include most of the land-dwelling vertebrates alive today, namely, mammals, turtles, sphenodons, lizards,
crocodilians and birds. It is a diverse clade with over 20000 living species. Amniotes include nearly all of the large plant- and flesh-eating vertebrates on land today, and they live all over the planet in virtually every habitat. They also sport disparate shapes - chameleons, bats, walruses, Homo sapiens, soft-shelled turtles, ostriches and snakes are but a few examples - and they include some of the smallest (sphaerodactyline geckoes) and largest (mysticete whales) vertebrates. Although fundamentally land dwellers, several clades such as ichthyosaurs, plesiosaurs, pinnipeds and cetaceans have returned to the sea. A few forms are gliders - including the Flying Dragon lizards, the Sugar Glider, Flying Squirrels and some gliding snakes - and powered aerial flight has originated three separate times, first in pterosaurs, then in birds, and later still in bats.
An extensive fossil record documents the origin and early evolution of Amniota, and that record has played a key role in understanding phylogenetic relationships among the living amniotes. The oldest amniotes currently known date from the Middle Pennsylvanian locality known as Joggins, in Nova Scotia. The relationships of these fossils indicate that amniotes first diverged into two lines, one line (Synapsida) that culminated in living mammals, and another line (Sauropsida) that embraces all the living reptiles (including birds). One Joggins fossil, the "protorothyridid" Hylonomus, appears to be a very early member of the line leading to Sauria (Crown-clade diapsids), the clade encompassing all living diapsids. This suggests that the more inclusive clade of which turtles (Testudines) are part (Anapsida) in most morphological phylogenies had diverged as well, even though its current record extends back only to the Lower Permian. An older amniote (from the Lower Carboniferous) was reported, However more recent studies suggested that it was only a close relative of amniotes, and the latest study even suggested that it was more likely to be a stem-tetrapod or an early amphibian than a relative of amniotes.