Northern Bluefin Tuna, Thunnus thynnus
Vertebrates are animals belonging to the subphylum Vertebrata, that is, animals with backbones or spinal columns. Additional defining characteristics of the subphylum are a muscular system that mostly consists of paired masses, as well as a central nervous system that is partly located inside the backbone. The name of this group comes from the bones of the spinal column (or vertebral column), which are called vertebrae.
Vertebrata is the largest subphylum of the phylum Chordata (chordates), and includes animals with which many people are familiar. Fish (including lampreys), amphibians, reptiles, birds, and mammals (including humans) are vertebrates. Over 50,000 species of vertebrates have been described. However, more than 95 percent of described animal species are invertebrates—a disparate classification of all animals without backbones.
In general, people feel a strong affinity toward vertebrates. Although invertebrates greatly outnumber vertebrates, it is overwhelmingly vertebrates that are kept as pets or in aquariums and terrariums, as well as being featured in movies, animated films, and other media. Communities pass laws relating to the humane treatment of vertebrates, and they build zoos for their display. The more similar the class of vertebrates is to humans, generally the more the apparent attraction and the more laws for their protection—as seen in the special attention shown to birds and especially mammals, versus fish, amphibians, and reptiles. (For example, some universities do not permit research on any vertebrates other than fish, amphibians, and reptiles.) This may be a reflection of the philosophical perspective that people can best love and feel joy from those beings that can most mirror their own character and form.
The study of animals with backbones is called vertebrate zoology. Vertebrate paleontology is the study of animals with backbones through their fossilized remains, and includes studies aimed at connecting animals of the past with modern day relatives.
The internal skeleton that defines vertebrates consists of cartilage or bone, or in some cases both. It is speculated that the first bony substance that vertebrates evolved was an outer skeleton in the form of a bony armor, and that its primary function was as a phosphate reservoir, excreted as calcium phosphate and stored around the body, offering protection at the same time. The internal skeleton provides support to the organism during the period of growth. For this reason, vertebrates can achieve larger sizes than invertebrates, and on average vertebrates are, in fact, larger. The skeleton of most vertebrates, excluding the most primitive ones, consists of a skull, the vertebral column, and two pairs of limbs. In some forms of vertebrates, one or both of these pairs of limbs may be absent, such as in snakes or whales. These limbs are assumed to have been lost during the course of evolution.
The skull is thought to have facilitated the development of intelligence as it protects vital organs such as the brain, the eyes, and the ears. The protection of these organs is also thought to have positively influenced the development of the high responsiveness to the environment often found in vertebrates.
Both the vertebral column and the limbs offer overall support to the body of the vertebrate. This support facilitates movement, which is normally achieved with muscles that are attached directly to the bones or cartilages. The muscles form the contour of the body of a vertebrate. A skin covers the inner parts of a vertebrate's body. The skin sometimes acts as a structure for protective features, such as horny scales or fur. Feathers may also be attached to the skin.
The trunk of a vertebrate houses the internal organs. The heart and the respiratory organs are protected in the trunk. The heart is located either behind the gills, or, in air-breathing vertebrates, between the lungs.
The central nervous system of a vertebrate consists of the brain and the spinal cord. In lower vertebrates, the brain mostly controls the functioning of the sense organs. In higher vertebrates, the size of the brain relative to the size of the body generally is greater. This larger brain enables more intensive exchange of information between the different parts of the body. The nerves from the spinal cord, which lies behind the brain, extend to the skin, the inner organs, and the muscles. Some nerves are directly connected to the brain, linking the brain with the ears and lungs.
A 2012 article in the scholarly publication Science identified the world's smallest known vertebrate as the frog species Paedophryne amauensis, with adults reaching a average size of 7.7 millimeters in length. Its discovery in a rainforest in eastern New Guinea pushed into second place an Indonesian fish from the carp family, with the adult female fish growing to about 7.9 millimeters (Lee 2012).
The world's largest vertebrate is the blue whale (Balaenoptera musculus), which reaches up to 33 meter (110 ft) in length and 181 metric ton (200 short tons) or more in weight.
Vertebrates are the best known among the animals, with most species having been identified and described. There are comprehensive listings of the extant (living) species of mammals, birds, reptiles, amphibians, and fish.
Nonetheless, the determination of the number of species of vertebrates is necessarily imprecise. One reason is that taxonomists generally strive to arrange species based on evolutionary relationships. As more insights are obtained, there are taxonomic rearrangements and new nomenclature, even to the point that sometimes species are reclassified as subspecies, and vice-versa. For example, in herpetological classifications, dealing with reptiles and amphibians, the adoption of the evolutionary species concept, versus the previously used biological species concept, led to the elevation of many subspecies to species status (Uetz 2000). Molecular studies are expected to lead to additional rearrangements. A second reason why it is difficult to determine the exact number of species is that new species are continually being discovered and described. Fish are being described at a rate of about 200 per year, amphibians at the rate of about 80 species per year, and reptiles at the rate of about 60 species per year (Eschmeyer, Ferraris, and Hoang 1998; Bauer 1999; Glaw and Kohler 1998; Uetz 2000).
According to a report by Uetz in 2000, comprehensive compilations of vertebrates reveal a species total of 4,675 mammals, 9,702 birds, 7,870 reptiles, 4,780 amphibians, and 23,250 fishes. Of the reptiles, the majority were determined to be lizards (4,470 species) and snakes (2,920). Over one half of all reptile species fall into the category of either colubrid snakes (approximately 1,800 species), skinks (1,200 species), or geckos (1,000 species). A subsequent tabulation by Uetz in 2005 showed a total of 8,240 extant reptile species.
A 2004 list of species by the International Union for the Conservation of Nature and Natural Resources (IUCN) yielded the following number of described species of vertebrates: mammals (5,416), birds (9,917), reptiles (8,163), amphibians (5,743), and fishes (28,500). This totals to 57,739 identified vertebrate species. Meanwhile, the IUCN reports a total of 1,190,200 described, extant species of invertebrates (although this represents an assumably small proportion of actual species of this very incompletely known group).
Vertebrates have been traced back to the ostracoderms (primitive jawless fish) of the Silurian Period (444 million to 409 million years ago) and the conodonts, a group of eel-like vertebrates characterized by multiple pairs of bony toothplates. Vertebrates started to evolve about 530 million years ago during the Cambrian explosion.
Vertebrates (subphylum Vertebrata) are part of the phylum Chordata, which are animals that had, at some stage in their life, a notochord, a hollow dorsal nerve cord, and pharyngeal slits, among other characteristics. Chordata includes two subphyla of invertebrates (Urochordata and Cephalochordata) and the Vertebrates.
Vertebrates are also considered part of the Craniata, a group of animals that includes all animals with skulls. Craniata consists of the vertebrates and hagfish (Myxini). In some taxonomies, hagfish, which lack vertebrae, nonetheless are included in Vertebrata based on presumed evolutionary relatedness.
Vertebrates are generally divided into two major groups: Agnatha (jawless vertebrates) and Gnathostomata (jawed vertebrates). The tetrapods (amphibians, reptiles, birds, and mammals) are placed in Gnathostomata, as well as those fish with hinged jaws. The lampreys (Petromyzontiformes) are placed in Agnatha, as well as several extinct orders of jawless vertebrates. Hagfish are generally classified in Agnatha, despite the lack of vertebrae. In some classification schemes, Agnatha and Gnathostomata are considered superclasses, and in other schemes Agnatha is considered a class.
Traditionally, seven classes of extant (living) vertebrates are recognized, three of fish and four of tetrapods:
However, the classification of vertebrates by Benton (2004), summarized below, recognizes two different classes of reptiles: († extinct)
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