Traditional versus Phylogenetic classification of Vertebrata
Vertebrates are those animals that have a backbone. The traditional system (right-hand column) recognizes seven taxonomic Classes of living Vertebrata: Agnatha [jawless "fish"], Chrondrichthyes [cartilaginous "fish"], Osteichthyes [bony fish], Amphibia [scaleless tetrapods], Reptilia [scaly tetrapods], Aves [feathered bipeds], and Mammalia [hairy tetrapods]. Note that many of these classes are defined by "absence" characters ("jawless," "scaleless"). [An eighth class of early, jawed fish-like creatures, Placodermi, is wholly extinct]
In the phylogenetic system (branching tree), taxonomic groups are defined by shared evolutionary characters (indicated by red crossbars) that defined a series of nested groups. For example, mammals (Mammalia) are an evolutionary lineage within Vertebrata [presence of backbone] that is defined successively as Gnathostomata [hinged, opposable jaws], Tetrapoda [four pentadactyl limbs], Amniota [amniotic egg membrane], and Synapsida [single temporal opening in the skull]. Note that the traditional recognition character "hair" for living mammals is not used, because early mammals lacked hair. "Reptiles" and "Aves" are both classified as Diapsida [two temporal openings in the skull], and "Fish" are separated into several groups, including the "fleshy-finned" Sarcoptyergia, which are more closely related to terrestrial tetrapods.
The phylogenetic system accurately reflects the evolutionary history of Vertebrates. The traditional system reflects certain impressions about shared similarities among organisms, which may be misleading. For example, hagfish and lampreys resemble each other in being "jawless" [as are redwood trees], where the hagfish lineage separated before the evolution of jaws, and the lamprey lineages originally had jaws but lost them as an adaptation for their parasitic lifestyle. They are otherwise quite distinct: for example, hagfish do not have a true backbone, and the braincase of lampreys resembles that of the gnathostomes. Early tetrapods had scales, and the three living orders of "scaleless" amphibia probably lost scales independently for similar adaptive reasons.