Vertebrate

Vertebrates; Temporal range: 525–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N Ordovician - Recent
	Individual organisms from each vertebrate class. Clockwise, starting from top left: Fire Salamander, Saltwater Crocodile, Southern Cassowary, Black-and-rufous Giant Elephant Shrew, Ocean Sunfish
Scientific classification
Domain:	Eukarya
Kingdom:	Animalia
Phylum:	Chordata
(unranked)	Craniata
Subphylum:	Vertebrata; Cuvier, 1812
Simplified grouping (see text)
	Fishes; Tetrapodes;

Vertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described.^[1] Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony fish, sharks and rays, amphibians, reptiles, mammals, and birds. Extant vertebrates range in size from the carp species Paedocypris, at as little as 7.9 mm (0.3 inch), to the Blue Whale, at up to 33 m (110 ft). Vertebrates make up about 5% of all described animal species; the rest are invertebrates which lack backbones.

The grouping sometimes includes the hagfish, which have no vertebrae, but are genetically quite closely related to lampreys, which do have vertebrae.^[2] For this reason, the sub-phylum is sometimes referred to as "Craniata", as all members do possess a cranium.

Etymology

The word vertebrate derives from Latin vertebrātus (Pliny), meaning having joints.^{[citation needed]} It is closely related to the word vertebra, which refers to any of the bones or segments of the spinal column.^[3]

Anatomy and morphology

All vertebrates are built along the basic Chordate body plan: A stiff rod running through the length of the animal (vertebral column or notochord, with a hollow tube of nervous tissue (the spinal cord) above it and the gastrointestinal tract below. In all vertebrates the mouth is found at or right below the anterior end of the animal, while the anus opens to the exterior before the end of the body. The remaining part of the body continuing aft of the anus form a tail with vertebrae and spinal cord, but no gut.

The defining characteristic of a vertebrate is the vertebral column, in which the notochord (a stiff rod of uniform composition) has been replaced by a segemented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs, derived embryonicly and evolutionarily from the notochord). However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, as in the sturgeon. Jawed vertebrates are typified by pair appendages (fins or legs, which may be secondarily lost), but this is not part of the definition of vertebrates as a whole.

Evolutionary history

Vertebrates originated about 525 million years ago during the Cambrian explosion, which is part of the Cambrian period. The earliest known vertebrate is Myllokunmingia.^[4] According to recent molecular analysis Myxini (hagfish) also belong to Vertebrata. Others consider them a sister group of vertebrates in the common taxon of Craniata.^[2] Another early vertebrate is Haikouichthys ercaicunensis, also from the Chengjiang fauna 518 million years ago. All of these groups lacked a jaw in the common sense.

Jawed vertebrates appeared in the Ordovician, and became common in the Devonian, the "Age of Fishes". The Devonian also saw the demise of much of the early jawless forms as well as the rise of the first labyrinthodonts, transitional between fish and amphibians.

The reptiles appeared in the subsequent Carboniferous period. The anapsid and synapsid reptiles where common during the late Paleozoic, while the diapsids became dominant during the Mesozoic. The dinosaurs gave rise to the birds in the Jurassic. The demise of the dinosaurs at the end of the Cretaceous promoted expansion of the mammals, which had developed from the synapsid reptiles during the Late Triassic Period.

Taxonomy and classification

There are several ways of classing animals. Evolutionary systematics relies on anatomy, physiology and evolutionary history. Phylogenetic classification is based solely on phylogeny. Evolutionary systematics give an overview; phylogenetic systematics gives detail. The two systems are thus complementary rather than opposed.^[5]

Formal classification

Traditional classification has the vertebrates grouped into seven classes based on gross anatomical and physiological traits. This classification is the one most commonly encountered in school textbooks, overviews, non-specialist and popular works. ^[6]

Superclass Agnatha (jawless fish)
Class Chondrichthyes (cartilaginous fish)
Class Osteichthyes (bony fish)
Class Amphibia (amphibians)
Class Reptilia (reptiles)
Class Aves (birds)
Class Mammalia (mammals)

Most of the classes listed are not "complete" taxons: the agnathans have given rise to the jawed vertebrates; the cartilaginous fishes have given rise to the bony fishes, which in turn have given rise to the land vertebrates. On land the amphibians gave rise to the reptiles and the reptiles to both birds and mammals.

Phylogenetic classification

While the above classification is orderly, it has come under critique from cladistics, as most of the groups are paraphyletic, i.e. have given rise to other groups. Quite a few authors working in the field use a classification based on purely on phylogeny, organized by their evolutionary history and disregarding the anatomy and physiology. An example based on Janvier (1981, 1997), Shu et al.. (2003), and Benton (2004)^[7] is given here:

Subphylum Vertebrata
- (Unranked group) Hyperoartia (lampreys)
- Class †Conodonta
- Class †Pteraspidomorphi
- Class †Thelodonti
- Class †Anaspida
- Class †Galeaspida
- Class †Pituriaspida
- Class †Osteostraci
- Infraphylum Gnathostomata (jawed vertebrates)

Class †Placodermi (Paleozoic armoured forms)
Class Chondrichthyes (cartilaginous fish)
Class †Acanthodii (Paleozoic "spiny sharks")

Superclass Osteichthyes (bony fish)

Class Actinopterygii (ray-finned fish)
Class Sarcopterygii (lobe-finned fish)

Subclass Coelacanthimorpha (coelacanths)
Subclass Dipnoi (lungfish)
Subclass Tetrapodomorpha (ancestral to tetrapods)

Superclass Tetrapoda (four-limbed vertebrates)

Class Amphibia (amphibians)

Series Amniota (amniotic embryo)

Class Sauropsida (reptiles and birds)

Class Aves (birds)

Subclass Synapsida (mammal-like reptiles)

Class Mammalia (mammals)

References

^ Jonathan E.M. Baillie; et al. (2004). "A Global Species Assessment". World Conservation Union. {{cite web}}: Explicit use of et al. in: |author= (help)
^ ^a ^b Kuraku; et al. (December 1999). "Monophyly of Lampreys and Hagfishes Supported by Nuclear DNA–Coded Genes". Journal of Molecular Evolution doi:10.1007/PL00006595. 49: 729. doi:10.1007/PL00006595. {{cite journal}}: Explicit use of et al. in: |author= (help); External link in |journal= (help); templatestyles stripmarker in |journal= at position 32 (help)
^ Douglas Harper, Historian. "vertebra". Online Etymology Dictionary. Dictionary.com.
^ Shu; et al. (November 4 1999). "Lower Cambrian vertebrates from south China". Nature. 402: 42–46. doi:10.1038/46965. {{cite journal}}: Check date values in: |date= (help); Explicit use of et al. in: |author= (help)
^ Hildebran, M. & Gonslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. John Wiley & Sons, Inc. New York, page 33: Comment: The problem of naming sister groups
^ Romer, A.S. (1949): The Vertebrate Body. W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)
^ Benton, Michael J. (2004-11-01). Vertebrate Palaeontology (Third ed.). Blackwell Publishing. pp. 455 pp. ISBN 0632056371/978-0632056378. {{cite book}}: Check |isbn= value: invalid character (help)

Bibliography

Kardong, Kenneth V. (1998). Vertebrates: Comparative Anatomy, Function, Evolution (second ed.). USA: McGraw-Hill. pp. 747 pp. ISBN 0-07-115356-X/0-697-28654-1. {{cite book}}: Check |isbn= value: invalid character (help)
"Vertebrata". Integrated Taxonomic Information System. 6 August. {{cite web}}: Check date values in: |date= and |year= / |date= mismatch (help)

External links

Tree of Life
Tunicates and not cephalochordates are the closest living relatives of vertebrates
Vertebrate Pests chapter in United States Environmental Protection Agency‎ and University of Florida/Institute of Food and Agricultural Sciences National Public Health Pesticide Applicator Training Manual

[1] Jonathan E.M. Baillie; et al. (2004). "A Global Species Assessment". World Conservation Union. {{cite web}}: Explicit use of et al. in: |author= (help)

[hagfish-2] Kuraku; et al. (December 1999). "Monophyly of Lampreys and Hagfishes Supported by Nuclear DNA–Coded Genes". Journal of Molecular Evolution doi:10.1007/PL00006595. 49: 729. doi:10.1007/PL00006595. {{cite journal}}: Explicit use of et al. in: |author= (help); External link in |journal= (help); templatestyles stripmarker in |journal= at position 32 (help)

[3] Douglas Harper, Historian. "vertebra". Online Etymology Dictionary. Dictionary.com.

[4] Shu; et al. (November 4 1999). "Lower Cambrian vertebrates from south China". Nature. 402: 42–46. doi:10.1038/46965. {{cite journal}}: Check date values in: |date= (help); Explicit use of et al. in: |author= (help)

[5] Hildebran, M. & Gonslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. John Wiley & Sons, Inc. New York, page 33: Comment: The problem of naming sister groups

[6] Romer, A.S. (1949): The Vertebrate Body. W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)

[Benton2004-7] Benton, Michael J. (2004-11-01). Vertebrate Palaeontology (Third ed.). Blackwell Publishing. pp. 455 pp. ISBN 0632056371/978-0632056378. {{cite book}}: Check |isbn= value: invalid character (help)

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