The amniotes are a group of tetrapod vertebrates that have a terrestrially adapted egg. They currently include mammals (synapsids), and sauropsids (reptiles and birds), as well as their fossil ancestors. 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.The first amniotes (referred to as "basal amniotes" or "stem amniotes"), such as Casineria, resembled small lizards and evolved from amphibian reptiliomorphs about 340 million years ago. Their eggs could survive out of the water, allowing amniotes to branch out into drier environments. The eggs could also "breathe" and cope with waste, allowing the eggs and the amniotes themselves to evolve into larger forms.

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