amnioteista

Amniotes are a clade of tetrapod vertebrates distinguished by their amniotic egg, a self-contained water-conserving device that encloses the developing embryo with four extraembryonic membranes: the amnion, chorion, allantois, and yolk sac. This adaptation permits reproduction on dry land and reduces dependency on freshwater habitats. Adults typically have watertight skin reinforced with keratin and well-developed lungs, often with a supportive rib cage.

The two principal lineages within Amniota are Sauropsida (reptiles and birds) and Synapsida (mammals and their extinct relatives). The last common ancestor of amniotes lived in the late Carboniferous period, about 312 million years ago. From these lineages, the diversity of modern reptiles, birds, and mammals arose through multiple radiations.

Amniotes exhibit a range of reproductive strategies, including oviparity, ovoviviparity, and viviparity. The placental mammal lineage developed extensive maternal-fetal exchange, while birds and many reptiles retain eggs with protective membranes. Some amniotes lay eggs with hard or leathery shells; others retain eggs internally or give birth to live young.

Fossil and molecular data place amniotes as a major evolutionarily successful group that enabled terrestrial colonization and ecological diversification. Early amniotes such as Hylonomus appear in the fossil record during the early Pennsylvanian, with the divergence of synapsids and sauropsids soon thereafter. Today amniotes comprise tens of thousands of species across diverse habitats.