Amniota, a group of limbed vertebrates that includes all living reptiles (class Reptilia), birds (class Aves), mammals (class Mammalia), and their extinct relatives and ancestors.

What do all amniotes have in common?

Characteristics of Amniotes

All amniotes have three membranes surrounding the fetus of one offspring. These membranes are the amnion, or protective layer, the top chorion layer, and the waste-absorbing allantois. These layers can be seen in the image of a chicken egg, below.

Do all amniotes have eggs?

Because reptiles, birds, and mammals all have amniotic eggs, they are called amniotes. The duck-billed platypus and some other mammals also lay eggs. But most mammals have evolved amniotic eggs that develop inside the mother's womb, or uterus, and so lack a shell.