The initial split was into synapsids (mammals and their relatives) and sauropsids (reptiles and birds). Mammals branched off from other amniotes very early. Several mammalian characteristics are ...

In several reptile lineages (except anapsids), openings developed in the solid dermal skull ... This condition originated in the reptilian ancestors of mammals and continues in mammals today. Diapsid ...

Mammal lineages are in red; diapsid reptiles, shown as archosaurs (birds, crocodilians and dinosaurs), are in blue; and lepidosaurs (snakes, lizards and relatives) are in green.