Amniotes are a clade of tetrapod vertebrates comprising the reptiles, birds, and mammals. Amniotes are characterized by having an egg equipped with an amnion, an adaptation to lay eggs on land or retain the fertilized egg within the mother.

Do all tetrapods have an amniotic egg?

Although some modern amphibians lay eggs on land, all amphibians lack advanced traits like an amnion. The amniotic egg formed through a series of evolutionary steps.

What species have amniotic eggs?

Birds, reptiles, and mammals have amniotic eggs. Because amphibian eggs don't have an amnion, the eggs would dry out if they were laid on the land, so amphibians lay their eggs in water. The larvae of most amphibians have gills and look like fish when they are born.