A vertebrate is any organism from the phylum chordata which specifically exhibits a backbone structure. To date approximately 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 cyclostomes, bony fish, sharks, rays, amphibians, reptiles, mammals, and birds. Living vertebrates range in size from a carp species in the genus Paedocypris, only 7.9 millimeters long, to the Blue Whale , at up to 33 meters. Vertebrates make up about one twentieth of all described animal species.
Vertebrates are characterized chiefly by their distinctinctive vertebrae column along what is considered the length of the animal, within which is a hollow tube of nervous tissue. In all vertebrates the mouth is found at or right below the anterior of this column. Only in higher vertebrates is there a significant enlargement of this nervous system central bundle to form an exceedingly large brain. Even though the superficial comparison of most vertebrate spinal morphologies shows great similarity, neurological studies show important differences in axon and stimulus control between the higher and lower vertebrate life forms. Evolutionary theory suggests the importance of a well developed and protected spinal nerve bundle was intended to provide considerable feedback and control between the mastication functions of the mouth and the muscle motor control functions of apendages used in gathering food.
Extinct fish, Hypsospondylus, class Actinopterygii. Source: Museo di storia Naturale di Milano Vertebrates appeared approximately 525 million years ago during the Cambrian explosion, an event of massive rise in species diversity. The Cambrian vertebrates were primitive in anatomical development, lacking jaw structures, but manifesting a notocord and a rudimentary vertebrae structure.
The first jawed vertebrates appeared in the Ordivician Period and then 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 the jawless fishes, except 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 close of the Devonian Period.
The first reptiles originated out of labyrinthodonts in the Carboniferous period. Both anapsid and synapsid reptile groups were abundant in the latter Paleozoic , although diapsids were a more widespread within the Mesozoic; however, within oceans, rivers and lakes, bony fishes became preponderant. Avifauna derived from dinosaurs within the Jurassic. At the Cretaceous Period terminus, there was a mammalian explosion, evolving from therapsids at the late Triassic, which therapsids were a set of reptilian synapsid reptiles.
Extinct Devonian shark Cladoselache. Source: Antique public domain print Vertebrate taxonomy has become controversial and disputed in the last two decades. The classical taxonomy utilized for vertebrate is:
- Class Agnatha (jawless fish)
- Class Chondrichthyes (cartilaginous fishes)
- Class Osteichthyes (bony fishes)
- Class Amphibia (amphibians)
- Class Reptilia (reptiles)
- Class Aves (birds)
- Class Mammalia (mammals)
The situation became more complex as scientists realized that many birds and mammals have reptilian ancestors. Thus a purely monophyletic group (a group of organisms which consists of an ancestor and all its descendants) would include combinations of the classical groups.
- ^ Jonathan E.M. Baillie, et al. 2004. A Global Species Assessment World Conservation Union
- ^ H.G.J.M. Kuypers and G.F.Matin. 1982. Anatomy of descending pathways to the spinal cord. Elsevier. 411 pages
- ^ M.Hildebran & G.Gonslow. 2001. Analysis of Vertebrate Structure. 5th edition. John Wiley & Sons, Inc. New York