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Temporal range: Cambrian-Recent,[1] 525–0 Ma

Fire salamander, saltwater crocodile, southern cassowary, black-and-rufous giant elephant shrew, ocean sunfish

Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Craniata
Subphylum: Vertebrata
Lamarck, 1801[2]
Simplified grouping (see text)

Ossea Batsch, 1788[2]

Vertebrates /ˈvɜrtɨbrəts/ are animals that are any species of animals within the subphylum Vertebrata /-ɑː/ (chordates with backbones). Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 64,000 species described.[3] Vertebrates include the jawless fish and the jawed vertebrates, which includes the cartilaginous fish (sharks and rays) and the bony fish. A bony fish clade known as the lobe-finned fishes is included with tetrapods, which are further divided into amphibians, reptiles, mammals, and birds. Extant vertebrates range in size 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 about 4% of all described animal species; the rest are invertebrates, which lack vertebral columns.

The vertebrates traditionally include the hagfish, which do not have proper vertebrae, though their closest living relatives, the lampreys, do.[4] Hagfish do, however, possess a cranium. For this reason, the vertebrate subphylum is sometimes referred to as "Craniata" when discussing morphology. Molecular analysis since 1992 has suggested that the hagfish are most closely related to lampreys,[5] and so also are vertebrates in a monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of Craniata.[6]


The word vertebrate derives from the Latin word vertebratus (Pliny), meaning joint of the spine.[7] It is closely related to the word vertebra, which refers to any of the bones or segments of the spinal column.[8]

Anatomy and morphology[edit]


Vertebral column[edit]




Gill arches bearing gills in a pike


In amphibians and some primitive bony fishes, the larvae bear external gills, branching off from the gill arches.[13] These are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution of tetrapods.[14]


Central nervous system[edit]

The central nervous system of vertebrates is based on a hollow nerve cord running along the length of the animal. Of particular importance and unique to vertebrates is the presence of neural crest cells. These are progenitors of stem cells, and critical to coordinating the functions of cellular components.[15] Neural crest cells migrate through the body from the nerve cord during development, and initiate the formation of neural ganglia and structures such as the jaws and skull.[16][17][18]


A peripheral nervous system branches out from the nerve cord to innervate the various systems. The front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles: The prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain), further differentiated in the various vertebrate groups.[19] Two laterally placed eyes form around outgrows from the midbrain, except in hagfish, though this may be a secondary loss.[20][21] The forebrain is well developed and subdivided in most tetrapods, while the midbrain dominate in many fish and some salamanders. Vesicles of the forebrain are usually paired, giving rise to hemispheres like the cerebral hemispheres in mammals.[19]


Evolutionary history[edit]

First vertebrates[edit]

The early vertebrate Haikouichthys

Vertebrates originated about 525 million years ago during the Cambrian explosion, which saw the rise in organism diversity. The earliest known vertebrate is believed to be the Myllokunmingia.[1] Another early vertebrate is Haikouichthys ercaicunensis. Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail.[22] All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed.[23] A vertebrate group of uncertain phylogeny, small-eel-like conodonts, are known from microfossils of their paired tooth segments from the late Cambrian to the end of the Triassic.[24]

From fish to amphibians[edit]

Acanthostega, a fish-like early labyrinthodont.

The first jawed vertebrates appeared in the latest Ordovician and became common in the Devonian, often known as the "Age of Fishes".[25] The two groups of bony fishes, the actinopterygii and sarcopterygii, evolved and became common.[26] The Devonian also saw the demise of virtually all jawless fishes, save for lampreys and hagfish, as well as the Placodermi, a group of armoured fish that dominated much of the late Silurian. The Devonian also saw the rise of the first labyrinthodonts, which was a transitional form between fishes and amphibians.

Mesozoic vertebrates[edit]

The reptiles appeared from labyrinthodonts in the subsequent Carboniferous period. The anapsid and synapsid reptiles were common during the late Paleozoic, while the diapsids became dominant during the Mesozoic. In the sea, the bony fishes became dominant. The dinosaurs gave rise to the birds in the Jurassic.[27] The demise of the dinosaurs at the end of the Cretaceous allowed for expansion of the mammals, which had evolved from the therapsids, a group of synapsid reptiles, during the late Triassic Period.

After the Mesozoic[edit]




There are several ways of classifying animals. Evolutionary systematics relies on anatomy, physiology and evolutionary history, which is determined through similarities in anatomy and, if possible, the genetics of organisms. Phylogenetic classification is based solely on phylogeny.[28] Evolutionary systematics gives an overview; phylogenetic systematics gives detail. The two systems are thus complementary rather than opposed.[29]

Traditional classification[edit]

Traditional spindle diagram of the evolution of the vertebrates at class level



While this traditional classification is orderly, most of the groups are paraphyletic, i.e. do not contain all descendants of the class' common ancestor.[30] For instance, descendants of the first reptiles include modern reptiles as well as mammals and birds. Most of the classes listed are not "complete" (and therefore paraphyletic) taxa, meaning they do not include all the descendants of the first representative of the group. For example, the agnathans have given rise to the jawed vertebrates; the bony fishes have given rise to the land vertebrates; the traditional "amphibians" have given rise to the reptiles (traditionally including the synapsids, or mammal-like "reptiles"), which in turn have given rise to the mammals and birds. Most scientists working with vertebrates use a classification based purely on phylogeny, organized by their known evolutionary history and sometimes disregarding the conventional interpretations of their anatomy and physiology.

Phylogenetic relationships[edit]



Vertebrate = fishes (+ Tetrapoda)

Hyperoartia (lampreys)













Placodermi (armoured fishes)


Chondrichthyes (cartilaginous fishes)


Acanthodii (spiny sharks)


Actinopterygii (ray-finned fishes)



Actinistia (coelacanths)



Dipnoi (lungfishes)





See also[edit]


  1. ^ a b Shu et al. (4 November 1999). "Lower Cambrian vertebrates from south China". Nature 402 (6757): 42–46. Bibcode:1999Natur.402...42S. doi:10.1038/46965. 
  3. ^ Jonathan E.M. Baillie et al. (2004). "A Global Species Assessment". World Conservation Union. 
  7. ^ Douglas Harper, Historian. "vertebrate". Online Etymology Dictionary. 
  8. ^ Douglas Harper, Historian. "vertebra". Online Etymology Dictionary. 
  15. ^ Teng, L. and Labosky P. A. (2006). "Neural crest stem cells" In: Jean-Pierre Saint-Jeannet, Neural Crest Induction and Differentiation, pp. 206-212, Springer Science & Business Media. ISBN 9780387469546.
  16. ^ Gans C. and Northcutt R. G. (1983). "Neural crest and the origin of vertebrates: a new head". Science, 220 (4594): 268–273. doi:10.1126/science.220.4594.268
  17. ^ Bronner, M. E. and LeDouarin N. M. (2012). "Evolution and development of the neural crest: An overview". Developmental biology, 366 (1): 2–9. doi:10.1016/j.ydbio.2011.12.042. Full text
  21. ^ Hyperotreti - Hagfishes
  30. ^ a b Benton, Michael J. (1 November 2004). Vertebrate Palaeontology (Third ed.). Blackwell Publishing. pp. 33, 455 pp. ISBN 978-0632056378. 


External links[edit]