A vertebrate is any member of the taxonomic group Vertebrata, a subphylum of the phylum Chordata. The Vertebrata includes all species of chordates with backbones: that is, all fishes, amphibians, reptiles, birds, and mammals. In popular usage, however, the term vertebrate is often used more narrowly to mean only those members of the subphylum Craniata (the craniates), which include the jawed vertebrates: that is, fishes with jaws (including hagfishes), and tetrapods with jaws—amphibians, reptiles (including dinosaurs and crocodilians), birds (aves), and mammals.
The five major groups of extant vertebrates are shown in the cladogram on the right. In this cladogram:
– Fishes include both cartilaginous fishes (sharks and rays) and bony fishes.
– Amphibians include frogs and toads, salamanders, newts, and caecilians.
– Reptiles include turtles , snakes , lizards , amphisbaenids , tuataras , crocodiles , and dinosaurs . Snakes are sometimes grouped with lizards in a paraphyletic group called “squamates”. Amphisbaenids are limbless burrowing reptiles closely related to lizards; they are sometimes grouped with snakes in a clade called “Serpentes”. Tuatara look superficially like lizards but are actually very different; they too are sometimes included in “Squamata”.
– Birds are descendants of feathered theropod dinosaurs .
– Mammals emerged from synapsid reptilian ancestors . Marsupials (“pouched” mammals) appeared later than placentals (“non-pouched” mammals) in mammalian evolution; monotremes (“egg-laying” mammals) branched off even later from other mammal groups.”