Vertebrate

? Vertebrates
Northern Bluefin Tuna, Thunnus thynnus
Scientific classification
Kingdom: Animalia Phylum: Chordata (unranked) Craniata Subphylum: Vertebrata Cuvier, 1812

Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns. The bones of the spinal column (or vertebral column) are called vertebrae.

Additional characteristics of the subphylum are a muscular system that mostly consists of paired masses, as well as a central nervous system which is partly located inside the backbone. Defining characteristics of a vertebrate are a backbone or spinal cord, a brain case, and an internal skeleton.

Vertebrata is the largest subphylum of chordates, and contains most animals with which people are generally familiar (except insects). Fish (including lampreys, but traditionally not hagfish, though this is now disputed), amphibians, reptiles, birds, and mammals (including humans) are vertebrates. Over 50,000 species of vertebrates have been described. However, more than 95% of described animal species are invertebrates.

Characteristic of vertebrates

The internal skeleton which defines vertebrates consists of cartilage or bone, or in some cases both. An outer skeleton in form of a bony armour was the first bony substance the vertebrates evolved. It is possible its primary function was as a phosphate reservoir, excreted as calcium phosphate and stored around the body, offering protection at the same time. The 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, that is 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 have been lost in 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 high responsiveness to the environment often found in vertebrates.

Both the vertebral column and the limbs support the body of the vertebrate overall. This support facilitates movement. Movement is normally achieved with muscles that are attached directly to the bones or cartilages. The contour of the body of a vertebrate is formed by the muscles. 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 is hollow and houses the internal organs. The heart and the respiratory organs are protected in the trunk. The heart is located behind the gills, or where there are lungs, in between the lungs.

The central nervous system of a vertebrate consists of the brain and the spinal cord. Both of these are characterized by being hollow. 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 is greater. This larger brain enables more intensive exchange of information between the different parts of the brain. 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.

Number of species of vertebrates

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, a determination of the exact number of species is necessarily inexact. 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, the adoption of the evolutionary species concept (see species) versus the previously used biological species concept led to the elevation of many subspecies to species status (Uetz 2000). Molecular studies will lead to additonal rearragements from the moprhological analyses. A second reason that 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 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 Uetz in 2000, comprehensive compilations of vertebrates yielded 4,675 mammals, 9,702 birds, 7,870 reptiles, 4,780 amphibians, and 23,250 fish. Of the reptiles, the majority are lizards (4,470 species) and snakes (2,920). over one half of all reptile species "belong to one of three families: colubrid snakes (about 1,850 species), skinks (1,200 species), and geckoes (1,000 species)." (need to edit) A subsequent tablulation by Uetz in 2005 showed a total of 8,240 extant reptile species.

A 2004 list of species by the International conservation union (IUCN) reproted the number of described species of vertebrates as: mammals (5,416), birds (9,917), reptiles (8,163), amphibians (5,743), and fishes (28,500). this yiels a total number of identified vertebrate species of 57,739. Meanwhile, the IUCN reports a total of 1,190,200 described, extant species of invertebrates (although this represents an assumbedly small proporiton of actual species of this poorly known group).

Vertebrates have been traced back to the ostracoderms 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, which is part of the Cambrian period (first known vertebrate is Myllokunmingia).

Taxonomy

"This is directly from the fish article in the EP and needs to be varied: Vertebrates are generally classified into two groups, the Agnatha (jawless vertebrates) and the Gnathostomata (jawed vertebrates). The later group includes fish with hinged jaws, but also includes amphibians, reptiles, birds, and mammals (the tetrapods). In most taxonomies, Agnatha and Gnathostomata are each considered a superclass, although sometimes Agnatha is considered a class (see taxonomy).

The Agnatha, in addition to including the modern day lampreys (Petromyzontiformes) and hagfish (Myxiniformes), also includes several extinct orders.

The hagfish "Hagfish, while generally classified in Agnatha ("jawless") and as fish, actually lack vertebrae. "

Within Gnathostomata, several classes of fish are recognized. Two of these classes have living representatives, the Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish). In some taxonomies, Osteichthyes is considered a superclass.

Thus, essentially there are seven classes of extant (living) vertebrates, if Aganatha is a class: Agnatha, *** Class Chondrichthyes (cartilaginous fish)

• • • Class Osteichthyes (bony fish)
      • • • Class Amphibia (amphibians)
            • Class Reptiles
              • Class Aves (birds)
              • Class Mammalia (mammals)

However, sometimes there are other classifications. The following is given by Benton:

Classification after Benton, 2004 [1].

• SUBPHYLUM VERTEBRATA
  • Class 'Agnatha' (paraphyletic)
    • Subclass †Conodonta
    • Subclass Myxinoidea (hagfish) (not always placed as vertebrata)
    • Subclass Petromyzontidae (lampreys)
    • Subclass †Pteraspidomorphi
    • Order †Thelodonti
    • Order †Anaspida
    • Subclass unnamed
      • Order †Galeaspida
      • Order †Pituriaspida
      • Order †Osteostraci
  • Infraphylum Gnathostomata (jawed vertebrates)
    • Class †Placodermi
    • Class Chondrichthyes (cartilaginous fish)
    • Class †Acanthodii
    • Class Osteichthyes (bony fish)
      • Subclass Actinopterygii (ray-finned fish)
      • Subclass Sarcopterygii (lobe-finned fish)
        • Order Actinistia (coelacanths)
        • Superclass Tetrapoda (four-limbed vertebrates)
          • Class Amphibia (amphibians)
          • Series Amniota (amniotic embryo)
            • Class Sauropsida (non-synapsid reptiles)
              • Class Aves (birds)
            • Class Synapsida (mammal-like reptiles)
              • Class Mammalia (mammals)

References

ISBN links support NWE through referral fees

• Benton, M. J. (2004), Vertebrate Paleontology, 3rd ed. Blackwell Science Ltd
• Colbert, E.H. (1969), Evolution of the Vertebrates, 2nd ed., John Wiley and Sons Inc.

External links

• Tree of Life
• Vertebrate Zoology
• Vertebrate: individual or colony