Vertebrates (/ˈvɜːrtəbrɪts, -ˌbreɪts/)[3] are deuterostomal animals with bony or cartilaginous axial endoskeleton — known as the vertebral column, spine or backbone — around and along the spinal cord, including all fish, amphibians, reptiles, birds and mammals. The vertebrates consist of all the taxa within the subphylum Vertebrata (/ˌvɜːrtəˈbreɪtə/)[4] (chordates with backbones) and represent the overwhelming majority of the phylum Chordata, with currently about 69,963 species described.[5]
Extant vertebrates vary in body lengths ranging from the frog species Paedophryne amauensis, at as little as 7.7 mm (0.30 in), to the blue whale, at up to 33 m (108 ft). Vertebrates make up less than five percent of all described animal species; the rest are described as invertebrates, an informal polyphyletic group comprising all that lack vertebral columns, which include non-vertebrate chordates such as lancelets.
The vertebrates traditionally include the hagfish, which do not have proper vertebrae due to their loss in evolution,[6] though their closest living relatives, the lampreys, do.[7] Hagfish do, however, possess a cranium. For this reason, the vertebrate subphylum is sometimes referred to as Craniata or "craniates" when discussing morphology. Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys,[8] and so also are vertebrates in a monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of Craniata.[9]
The word vertebrate derives from the Latin word vertebratus (Pliny), meaning joint of the spine.[10] A similarly derived word is vertebra, which refers to any of the irregular bones or segments of the spinal column.[11]
All vertebrates are built along the basic chordate body plan: a rigid axial endoskeleton (vertebral column and/or notochord) running along the length of the animal,[12] dorsal to the gut tube, with a hollow dorsal nerve cord (the neural tube, which develops into the brain and spinal cord) running more dorsal to it. The endoskeleton continues beyond the anus and often forms an elongated tail (post-anal tail).[13] All vertebrates also possess pharyngeal arches, as well as an iodine-concentrating organ called the endostyle, which develops into the thyroid in adults.
With only one exception, the defining characteristic of all vertebrate is the vertebral column, in which the embryonic notochord found in all chordates is replaced by a segmented series of mineralized elements called vertebrae separated by fibrocartilaginous intervertebral discs, which are derived embryonically and evolutionarily from the notochord. Hagfish are the only extant vertebrate whose notochord persists and is not integrated/ replaced by the vertebral column.