Synapsida is a group of animals that includes mammals and their ancestors. They are characterized by having a single temporal fenestra, an opening in the skull behind the eyes, which distinguishes them from Diapsida, another major group of reptiles. Synapsids first appeared in the late Carboniferous period and evolved into various forms over millions of years. The most well-known synapsids are mammals, which evolved from earlier synapsid ancestors called therapsids. These ancestors displayed features that eventually led to the development of warm-bloodedness, fur, and live births in mammals. Synapsida played a crucial role in the evolution of life on Earth.