Amnioten

Amnioten, also called amniotes, are a clade of tetrapod vertebrates that includes reptiles, birds, and mammals. They are distinguished by the amniotic egg and its protective membranes—the amnion, chorion, and allantois—an adaptation that allows development on dry land with reduced dependence on aquatic environments. The term derives from the embryonic amnion, a defining feature of this group. Additional characteristics often associated with amniotes include keratinized skin and efficient lung ventilation supported by a more rigid rib cage.

The amniotes divide into two major lineages: Sauropsida, which leads to modern reptiles and birds, and Synapsida, the lineage that gave rise to mammals and their extinct relatives. In the fossil record, amniotes first appear in the late Carboniferous period, roughly 312 to 320 million years ago, with rapid diversification during the Permian. Early amniotes occupied a variety of ecological niches, from insectivory to herbivory, setting the stage for later radiations in both lineages.

Today, amniotes represent a large portion of terrestrial vertebrate biodiversity. Modern representatives include numerous species of reptiles and birds, as well as the mammalian lineage, which ranges from monotremes to marsupials and placentals. The success of amniotes is attributed to their reproductive strategy, which decouples development from aquatic environments and enables occupation of a wide range of habitats worldwide.