<P> Amniotes are tetrapods (descendants of four - limbed and backboned animals) that are characterised by having an egg equipped with an amnion, an adaptation to lay eggs on land rather than in water as the anamniotes (including frogs) typically do . Amniotes include synapsids (mammals along with their extinct kin) and sauropsids (reptiles and birds), as well as their ancestors, back to amphibians . Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes . In eutherian mammals (such as humans), these membranes include the amniotic sac that surrounds the fetus . These embryonic membranes and the lack of a larval stage distinguish amniotes from tetrapod amphibians . </P> <P> The first amniotes, referred to as "basal amniotes", resembled small lizards and evolved from the amphibian reptiliomorphs about 312 million years ago, in the Carboniferous geologic period . Their eggs could survive out of the water, allowing amniotes to branch out into drier environments . The eggs could also "breathe" and cope with wastes, allowing the eggs and the amniotes themselves to evolve into larger forms . </P> <P> The amniotic egg represents a critical divergence within the vertebrates, one enabling amniotes to reproduce on dry land--free of the need to return to water for reproduction as required of the amphibians . From this point the amniotes spread around the globe, eventually to become the dominant land vertebrates . Very early in the evolutionary history of amniotes, basal amniotes diverged into two main lines, the synapsids and the sauropsids, both of which persist into the modern era . The oldest known fossil synapsid is Protoclepsydrops from about 312 million years ago, while the oldest known sauropsid is probably Paleothyris, in the order Captorhinida, from the Middle Pennsylvanian epoch (c. 306--312 million years ago). </P> <P> Zoologists characterize amniotes in part by embryonic development that includes the formation of several extensive membranes, the amnion, chorion, and allantois . Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified epithelium (rather than first entering a feeding larval tadpole stage followed by metamorphosis, as amphibians do). In amniotes, the transition from a two - layered periderm to cornified epithelium is triggered by thyroid hormone during embryonic development, rather than by metamorphosis . The unique embryonic features of amniotes may reflect specializations of eggs to survive drier environments; or the massive size and yolk content of eggs may have evolved to allow direct development of the embryo to a larger size . </P>

The two major lineages that make up the living amniota are