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.

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 ...