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Vertebrates are members of the subphylum Vertebrata, chordates with backbone or spinal column. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostome, 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. 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 vertebratus (Pliny), meaning having joints. It is closely related to the word vertebra, which refers to any of the bones or segments of the spinal column.

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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 forms 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characteristic of a vertebrate is the vertebral column, in which the notochord (a stiff rod of uniform composition) has been replaced by a segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs, derived embryonically and evolutionarily from the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 paired 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 was an event of massive rise in organism diversity that occurred in the Cambrian period. The earliest known vertebrate is believed to be the Myllokunmingia.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. Another early vertebrate is Haikouichthys ercaicunensis, also from the Chengjiang fauna . All of these groups lacked a jaw in the common sense.

The first jawed vertebrates appeared in the Ordovician and became common in the Devonian, often known as the "Age of Fishes". The two groups of bony fishes, the actinopterygii and sarcopterygii, evolved and became common. The Devonian also saw the demise of virtually all jawless fishes, save for lampreys and hagfish, as well as the rise of the first labyrinthodonts, transitional between fish and amphibians. The Placodermi, a group of fishes that dominated much of the late Silurian and the majority of the Devonian period, also became extinct at the end of the Devonian.

The reptiles appeared in the subsequent Carboniferous period. The anapsid and synapsid reptiles were 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 therapsids, a group of synapsid reptiles, during the Late Triassic Period.

Classification

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. Evolutionary systematics gives an overview; phylogenetic systematics gives detail. The two systems are thus complementary rather than opposed.

Traditional classification

Conventional classification has living vertebrates grouped into seven classes based on traditional interpretations of gross anatomical and physiological traits. This classification is the one most commonly encountered in school textbooks, overviews, non-specialist, and popular works:



While this traditional classification is orderly, it is frequently considered to be inaccurate phylogenetically, as most of the groups are paraphyletic, i.e. do not contain all descendants of the class's common ancestor. Descendants of the first reptiles do for instance include the birds and mammals as well as reptiles. Quite a few 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. An example based on Janvier (1981, 1997), Shu et al. (2003), and Benton (2004) is given here:



Most of the classes listed are not "complete" taxa: 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 mammal-like "reptiles"), which in turn have given rise to the birds and mammals.

Phylogenetic relationships

In phylogenetic taxonomy, the relationships between animals are not typically divided into ranks, but illustrated as a nested "family tree" known as a cladogram. Phylogenetic groups are given definitions based on their relationship to one another, rather than purely on physical traits such as the presence of a backbone. This nesting pattern is often combined with traditional taxonomy (as above), in a practice known as evolutionary taxonomy.

The cladogram presented below is based on studies compiled by Philippe Janvier and others for the Tree of Life Web Project.

See also



References

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