structural:scholar_edition, information:extraction
Modern classification

With the basic classification of tetrapods settled, a half a century followed where the classification of living and fossil groups was predominantly done by experts working within classes. In the early 1930s, American vertebrate palaeontologist Alfred Romer (1894–1973) produced an overview, drawing together taxonomic work from the various subfields to create an orderly taxonomy in his Vertebrate Paleontology. This classical scheme with minor variations is still used in works where systematic overview is essential, e.g. Benton (1998) and Knobill and Neill (2006). While mostly seen in general works, it is also still used in some specialist works like Fortuny et al. (2011). The taxonomy down to subclass level shown here is from Hildebrand and Goslow (2001):

This classification is the one most commonly encountered in school textbooks and popular works. While orderly and easy to use, it has come under critique from cladistics. The earliest tetrapods are grouped under class Amphibia, although several of the groups are more closely related to amniotes than to modern day amphibians. Traditionally, birds are not considered a type of reptile, but crocodiles are more closely related to birds than they are to other reptiles, such as lizards. Birds themselves are thought to be descendants of theropod dinosaurs. Basal non-mammalian synapsids ("mammal-like reptiles") traditionally also sort under class Reptilia as a separate subclass, but they are more closely related to mammals than to living reptiles. Considerations like these have led some authors to argue for a new classification based purely on phylogeny, disregarding the anatomy and physiology.