amniotic membranes
29.4 Reptiles The amniotes are distinguished from amphibians by the presence of a terrestrially adapted egg protected by amniotic membranes. The amniotes include reptiles, birds, and mammals. The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids mammals and sauropsids. Sauropsids can be further divided into anapsids turtles and diapsids birds and reptiles . Reptiles are tetrapods either having four limbs or descending from such. Limbless reptiles snakes are classified as tetrapods, as they are descended from four-limbed organisms. One of the key adaptations that permitted reptiles to live on land was the development of scaly skin containing the protein keratin, which prevented water loss from the skin. Reptilia includes four living clades: Crocodilia crocodiles and alligators , Sphenodontia tuataras , Squamata lizards and snakes , and Testudines turtles .