The initial split was into synapsids and sauropsids. Synapsids include all mammals, including extinct mammalian species. Synapsids also include therapsids, which were mammal-like reptiles from which mammals evolved. Sauropsids include reptiles and birds, and can be further divided into anapsids and diapsids.

Are birds synapsids?

Synapsids are a group of animals that includes mammals and every animal more closely related to mammals than to the other members of the amniote clade, such as reptiles and birds.

Are birds diapsid synapsid or synapsid reptiles?

So, this is the key difference between diapsid and synapsid. Most reptiles and all the birds are diapsids while most mammals are synapsids.