The class Reptilia, as traditionally defined, is paraphyletic because it excludes birds (class Aves) and mammals. Under the ranks of the ICZN Code, these three taxa are separate classes. However, mammals hail from the synapsids (which were once described as "mammal-like reptiles") and birds are sister taxon to a group of dinosaurs (part of Diapsida), both of which are "reptiles". Alternatively, reptiles are paraphyletic because they gave rise to (only) birds. Birds and reptiles together make Sauropsids, a clade of Amniota that is the sister group of the clade that includes mammals.