Amniotes is the group of reptiles, birds, and mammals. They all develop through an embryo that is enclosed within a membrane called an amnion. The amnion surrounds the embryo with a watery substance, and is probably an adaptation for breeding on land. This marine iguana for instance spends much of its time on the sea shore and lays its eggs on land.
Unlike the reptiles, amniotes are a monophyletic group.