The Synapsida was a great vertebrate group with a 300 million year history. They were the most successful reptiles during the Permian period and dominated the land vertebrate fauna of the Permian and early Triassic before losing ground to the diversifying dinosaurs and other lines of reptiles. During this decline, one surviving branch evolving from the therapsids were the mammal ancestors.
The fossil record provides conclusive evidence that synapsids were the first amniotes (animals who young developed in a protective sac) to diversify. Synapsids quickly became the most diverse, widespread and most common amniotes in the Late Carboniferous, and they maintained this predominant position throughout the Palaeozoic.