Final answer:
Birds, mammals, and reptiles are related through their shared ancestry as amniotes. Mammals evolved from synapsid amniotes, while birds are descended from diapsid amniotes, specifically theropod dinosaurs. This reflects a phylogenetic relationship that places birds within the reptile clade as highly modified diapsids.
Step-by-step explanation:
The question addresses the phylogenetic relationships among birds, mammals, and reptiles. A valuable framework for understanding these relationships is the concept of clades, which are used to illustrate evolutionary paths in cladograms.
According to current scientific consensus, birds and mammals branched off from different groups of the amniotes. Mammals emerged from the synapsid lineage, which evolved into therapsids and later into modern mammals.
In contrast, birds are derived from the diapsid lineage, more specifically, the archosaurs, which also include crocodilians, pterosaurs, and dinosaurs. Birds evolved directly from the theropod (maniraptoran) dinosaurs, with Archaeopteryx serving as a key transitional fossil linking birds and reptiles.
Despite traditional classifications previously placing birds in a separate class (Aves), overwhelming evidence supports that birds are actually highly modified diapsids and should be considered as part of the reptile clade.