Vertebrate

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Vertebrates Temporal range: Cambrian-Recent,[1] 525–0Ma PreЄ Є O S D C P T J K Pg N
Individual organisms from each major vertebrate group. Clockwise, starting from top left: Fire Salamander, Saltwater Crocodile, Southern Cassowary, Black-and-rufous Giant Elephant Shrew, Ocean Sunfish
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Craniata
Subphylum:	Vertebrata Cuvier, 1812
Simplified grouping (see text)
Fishes Tetrapods

Vertebrates /ˈvɜrtɨbrəts/ are animals that are members of the subphylum Vertebrata /-ɑː/ (chordates with backbones). Vertebrates include the overwhelming majority of the phylum Chordata, with currently about 64,000 species described.^[2] Vertebrates include the jawless fish, bony fish, sharks and rays, amphibians, reptiles, mammals, and birds. Extant vertebrates range in size from the frog species Paedophryne amauensis, at as little as 7.7 mm (0.3 inch), to the blue whale, at up to 33 m (110 ft). Vertebrates make up about 4% of all described animal species; the rest are invertebrates, which lack backbones.
The vertebrates traditionally include the hagfish, which do not have proper vertebrae, though their closest living relatives, the lampreys, do have vertebrae.^[3] 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,^[4] and so also are vertebrates in a monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of Craniata.^[5]

Contents

• 1 Etymology
• 2 Anatomy and morphology
  • 2.1 Vertebral column
  • 2.2 Gills
  • 2.3 Central nervous system
• 3 Evolutionary history
  • 3.1 First vertebrates
  • 3.2 From fish to amphibians
  • 3.3 Mesozoic vertebrates
  • 3.4 After the Mesozoic
• 4 Classification
  • 4.1 Traditional classification
  • 4.2 Phylogenetic relationships
• 5 See also
• 6 References
• 7 Bibliography
• 8 External links

Etymology

The word vertebrate derives from the Latin word vertebratus (Pliny), meaning joint of the spine.^[6] It is closely related to the word vertebra, which refers to any of the bones or segments of the spinal column.^[7]

Anatomy and morphology

See also: Mammal anatomy and Fish anatomy

All vertebrates are built along the basic chordate body plan: a stiff rod running through the length of the animal (vertebral column or notochord),^[8] 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.^[9]

Vertebral column

The defining characteristic of a vertebrate is the vertebral column, in which the notochord (a stiff rod of uniform composition) found in all chordates 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, such as the sturgeon^[10] and the Latimeria. 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.

Fossilized skeleton of Diplodocus carnegii, showing an extreme example of the backbone that characterizes the vertebrates. Exhibited at the Museum für Naturkunde (Museum of Natural Science), Berlin.

Gills

Gill arches bearing gills in a pike

All basal vertebrates breathe with gills. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the esophagus to the exterior. Each gill is supported by a cartilagenous or bony gill arch.^[11] The bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven. The vertebrate ancestor no doubt had more arches, as some of their chordate relatives have more than 50 pairs of gills.^[9]
In amphibians and some primitive bony fishes, the larvae bear external gills, branching off from the gill arches.^[12] These are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs in most amphibians. Some amphibans 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.^[13]
While the higher vertebrates do not have gills, the gill arches form during fetal development, and lay the basis of essential structures such as jaws, the thyroid gland, the larynx, the columella (corresponding to the stapes in mammals) and in mammals the malleus and incus.^[9]

Central nervous system

The vertebrates are the only chordate group to exhibit a proper brain. A slight swelling of the anterior end of the nerve cord is found in the lancelet, though it lacks the eyes and other complex sense organs comparable to those of vertebrates. Other chordates do not show any trends towards cephalisation.^[9]
The central nervous system is based on a hollow nerve tube running along the length of the animal, from which the peripheral nervous system branches out 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.^[14] Two laterally placed eyes form around outgrows from the midbrain, except in hagfish, though this may be a secondary loss.^[15][16] 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.^[14] The resulting anatomy of the central nervous system, with a single, hollow nerve cord topped by a series of (often paired) vesicles is unique to vertebrates. All invertebrates with well developed brains, like insects, spiders and squids have a ventral rather than dorsal system of ganglions, with a split brain stem running on each side of the mouth/gut.^[9]

Evolutionary history

See also: Evolution of fish and Evolution of tetrapods

First vertebrates

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.^[17] All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed.^[18] 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.^[19]

From fish to amphibians

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".^[20] The two groups of bony fishes, the actinopterygii and sarcopterygii, evolved and became common.^[21] 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 between fishes and amphibians.

Mesozoic vertebrates

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.^[22] The demise of the dinosaurs at the end of the Cretaceous promoted expansion of the mammals, which had evolved from the therapsids, a group of synapsid reptiles, during the late Triassic Period.

After the Mesozoic

The Cenozoic world has seen great diversification of bony fishes, frogs, birds and mammals.
Over half of all living vertebrate species (about 32,000 species) are fishes (non-tetrapod craniates), a diverse set of lineages that inhabit all the world's aquatic ecosystems, from snow minnows (Cypriniformes) in Himalayan lakes at elevations over 4,600 metres (15,000 feet) to flatfishes (order Pleuronectiformes) in the Challenger Deep, the deepest ocean trench at about 11,000 metres (36,000 feet). Fishes of myriad varieties are the main predators in most of the world’s water bodies, both freshwater and marine. The rest of the vertebrate species are tetrapods, a single lineage that includes amphibians (frogs, with more than 5,800 species; salamanders, with about 580 species; and caecilians, with about 175 species); mammals (with over 5,400 species); and reptiles and birds (with more than 18,000 species). Tetrapods dominate the megafauna of most terrestrial environments (including fossorial and arboreal realms) and also include many partially or fully aquatic groups (e.g., sea snakes, penguins, cetaceans).

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.^[23] Evolutionary systematics gives an overview; phylogenetic systematics gives detail. The two systems are thus complementary rather than opposed.^[24]