Outside of our fellow mammals, our next closest relatives are reptiles. As both birds and mammals are warm blooded (endothermic) and have four-chambered hearts, one might be tempted to think that the sister group to mammals would be birds. But the story is much more complicated than that, especially because birds are actually reptiles.

Reptiles include four main lineages: (1) turtles, (2) lizards and snakes, (3) crocodilians, and (4) dinosaurs, including birds. Indeed, birds are reptiles – birds are a surviving lineage descended from bipedal predatory dinosaurs! In decades past, there were five “classes” of vertebrates (animal groups with backbones): fishes, amphibians, mammals, reptiles, and birds. In fact, many basic treatments still list these groups. For example, Encyclopedia Britannica still has an article entitled: “Five Vertebrate Groups.” But there are major problems with two of these old groups: neither fishes nor scaly reptiles are monophyletic.