爬虫類は中生代に入って単弓類と入れ替わるようにして繁栄をはじめた

有羊膜類は、古生代石炭紀に哺乳類を含む単弓類の系統と、恐竜を含む爬虫類の系統に分岐した。特に、爬虫類の系統には眼窩の後ろに穴を持たない「無弓類」と、穴を２つ持つ「双弓類」が含まれる。メソサウルスなどの無弓類は古生代ペルム紀に繁栄したが、次の時代の中生代三畳紀には全て絶滅してしまった。

一方、双弓類は中生代に入って繁栄し、恐竜や翼竜を含む主竜類を生み出した。カメ類は眼窩の後ろに穴をもたないものの、最近の研究から双弓類に属することがわかってきた。双弓類にはそれら以外にも多様なトカゲ類が含まれ、現在も繁栄を続けている。

Reptiles began to replace Synapsids and flourished during the Mesozoic.

The Amniotes was composed by the Synapsid lineage, which includes mammals, and by the Reptile lineage including the dinosaurs. The reptile lineage is also divided in 2 classes: the Anapsids, which have no opening near the temples and the Diapsids, which have 2 openings. Anapsids such as Mesosaurus prospered during the Paleozoic Permian however they all went extinct during the Mesozoic Triassic period. When the Diapsids entered the Mesozoic, they prospered and gave birth to the Archosaur group that included the Dinosaurs and the Pterosaurs.

Although, even if turtles do not have holes at the back of their eye socket, recent research found that they belong to the Diapsid group. In addition to the turtles, the Diapsid group contains a large variety of lizards, and continues even now to prosper.