Amniotes are the group of the vertebrates that evolved from early tetrapod amphibians, and developed eggs suited to life in a terrestrial environment. They are believed to have evolved during the early Carboniferous period, perhaps around 340 million years ago. They are subclassified into various groupings based on the design of their skulls (or in some cases the skulls of their ancestors).

* Anapsids are amniotes with solid box-like skulls. Many early reptiles, like Scutosaurus were of this type. Additionally, it is possible, although it is not entirely clear whether turtles fall within this group - they may be anapsids (they have the right skull type), or they may be diapsids (see below) which re-evolved an anapsid-type skull.

* Diapsids are amniotes with two openings (temporal fenestra) on each side of their skull. They include lizards, snakes, the sphenadonts (the tuatara), dinosaurs, birds (even though they since evolved a heavily modified skull), crocodiles, pterosaurs, and a variety of extinct reptiles.

* Synapsids are amniotes with a single opening on each side of their skull. They include the mammals, and their relatives and ancestors, which are often known as "mammal-like reptiles".

The first mammal-like reptiles probably evolved around 320 million years ago. The last ones (other than mammals that is) are believed to have died out around 100 million years ago. For a while, during the Permian period, and the early Triassic, mammal-like reptiles were the dominant land creatures. There were many kinds of these animals, some of which look quite bizarre to us today - they include (among others) pelycosaurs such as Dimetrodon, which were equipped with a large sail on their back, and dinocephalians such as Estemmenosuchus, which had a strange array of horns on its head.