Amniotes are a clade of tetrapod vertebrates that comprise sauropsids (including reptiles and birds) and synapsids (including mammals). They are distinguished by a membrane (amnion) protecting the embryo and a lack of a larval stage.[5] Thanks to this, amniotes lay eggs on land or retain them within the mother, unlike anamniotes (fishes and amphibians), which typically lay eggs in water. Older sources, particularly before the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the antiquated idea of the evolutionary great chain of being. The term amniote comes from the Greek ἀμνίον amnion, "membrane surrounding the fetus", and earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός amnos, "lamb".[6]
The amnion comprises several extensive membranes. In eutherian mammals (such as humans), these include the amniotic sac that surrounds the fetus. The amnion is a critical divergence within vertebrates that allows the embryos to survive out of the water. This enabled amniotes to reproduce on land and so move into drier environments—free of the need to return to water for reproduction as amphibians. The eggs could also "breathe" and cope with wastes, allowing the eggs and individuals to evolve into larger forms.
The first amniotes, referred to as "basal amniotes", resembled small lizards and evolved from the amphibian reptiliomorphs about 312 million years ago,[7] in the Carboniferous geologic period. Amniotes spread around Earth's land and became the dominant land vertebrates.[7] They soon diverged into synapsids and sauropsids, which persist today. The oldest known fossil synapsid is Protoclepsydrops from about 312 million years ago,[7] while the oldest known sauropsid is probably Paleothyris, in the order Captorhinida, from the Middle Pennsylvanian epoch (c. 306–312 million years ago).
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 a cornified epithelium is triggered by thyroid hormone during embryonic development, rather than by metamorphosis.[8] The unique embryonic features of amniotes may reflect specializations for eggs to survive drier environments; or the increase in size and yolk content of eggs may have permitted, and coevolved with, direct development of the embryo to a large size.
Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell and an allantois that facilitates respiration while providing a reservoir for disposal of wastes. Their kidneys and large intestines are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures develop inside the placenta.