Page "Amniote" Paragraph 9

While the early amniotes resembled their amphibian ancestors in many respects, a key difference was the lack of an otic notch at the back margin of the skull roof.

In their ancestors, this notch held a spiracle, an unnecessary structure in an animal without an aquatic larval stage.

There are three main lines of amniotes, which may be distinguished by the structure of the skull and in particular the number of temporal fenestrae ( openings ) behind each eye.

In anapsids ( turtles ) there are none, in synapsids ( mammals and their extinct relatives ) there is one, and in most diapsids ( including birds, crocodilians, squamates, and tuataras ), there are two.