Synapsids

Synapsids are a group of animals that include mammals and their extinct relatives. They are characterized by a unique skull structure with a single opening behind each eye, called an temporal fenestra, which allowed for stronger jaw muscles. Originating over 300 million years ago, synapsids were dominant terrestrial vertebrates for millions of years before eventually giving rise to mammals. Today, mammals are considered the only surviving synapsids, but the group also includes many ancient, now-extinct species that played a key role in the evolution of land animals.