amniot

An amniot (amniote) is any member of the clade Amniota, a major lineage of tetrapod vertebrates characterized by embryonic development within an amniotic sac and associated membranes. Amniotes include mammals, birds, and reptiles, including turtles, lizards, snakes, and crocodilians, as well as many extinct groups. The defining adaptation is the amniotic egg, which houses the embryo in membranes—the amnion, chorion, and allantois—and is typically surrounded by a shell or shell-like covering that protects the embryo from desiccation. In addition, amniotes tend to have keratinized skin and other adaptations that reduce water loss, supporting terrestrial life.

Most amniotes reproduce with eggs, but many mammals are viviparous (live-bearing) and still retain amniotic membranes; in these species, the placenta replaces the egg's protective environment. The amniote lineage is divided into two major clades: Sauropsida (reptiles, turtles, birds) and Synapsida (mammals and their extinct relatives).

Amniotes first appeared during the late Carboniferous period, roughly 312 million years ago, and rapidly diversified. Today, amniotes occupy a wide range of habitats, from deserts to rainforests and from oceans to the skies. In summary, amniotes are a diverse group of land-adapted vertebrates defined by their embryonic development within an amniotic sac and their possession of extraembryonic membranes, enabling reproduction away from freshwater environments.