Synapsida are a group of vertebrates with one hole behind the eye socket. Synapsida include mammals and their ancestors. The first synapsid is the Archaeothyris, a lizard-like animal from the Carboniferous period. In the Mesozoic, the Synapsids played a minor role. After the extinction of the dinosaurs at the end of the Cretaceous period, Synapsids seized the opportunity and became the dominant animal group with a wide variety of mammals.