Vertebrata

Vertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. 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. 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 cyclostomes, 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 are about 5% of all animals. The rest are invertebrates.

Anatomy and morphology
Vertebrates are animals "with" a backbone. Invertebrates "do not" have backbones.

One characteristic of the subphylum are that all members have muscular systems that mostly consist of paired masses, as well as a central nervous system, which is partly located inside the backbone (if one is present). The defining characteristic of a vertebrate is considered the backbone or spinal cord, a brain case, and an internal skeleton, but the latter do not hold true for lampreys, and the former is arguably present in some other chordates. Rather, all vertebrates are most easily distinguished from all other chordates by having a clearly identifiable head, that is, sensory organs — especially eyes are concentrated at the fore end of the body and there is pronounced cephalization. Compare the lancelets, which have a mouth but not a well-developed head, and have light-sensitive areas along their entire back.

Evolutionary history
Vertebrates originated about 525 million years ago during the Cambrian explosion, which is part of the Cambrian period. The earliest known vertebrate is Myllokunmingia. According to recent molecular analysis Myxini (hagfish) also belong to Vertebrates. 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, 524 million years ago. All of these groups lacked a jaw in the common sense.

Jawed vertebrates appeared in the Ordovician, and became common in the Devonian, the "Age of Fishes". The Devonian also saw the demise of much of the early jawless forms as well as the rise of the first labyrinthodonts, transitional between fish and amphibians.

The reptiles appeared in the subsequent Carboniferous period. The anapsid and synapsid reptiles where 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 great dinosaurs at the end of the Cretaceous opened up for expansion of the mammals, who had developed from the synapsid reptiles.

Taxonomy and classification
There are several ways of classing animals. Traditional systematics or evolutionary systematics rely on anatomy, physiology and evolutionary history. Phylogenetic classification is based solely on phylogeny. Traditional systematics give overview, phylogenetic systematics give detail. The two systems are thus complementary rather than opposed.

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


 * Class Agnatha (jawless fish)
 * Class Chondrichthyes (cartilaginous fish)
 * Class Osteichthyes (bony fish)
 * Class Amphibia (amphibians)
 * Class Reptilia (reptiles)
 * Class Aves (birds)
 * Class Mammalia (mammals)

Note that most of the classes listed are not "complete" taxons: The agnathans have given rise to the jawed vertebrates, the cartilaginous fishes have given rise to the bony fishes, who in their turn have given rise to the land vertebrates. On land the amphibians gave rise to the reptiles and the reptiles to both birds and mammals.

Phylogenetic classification
While the above classification is orderly, it has come under critique from cladistics, as most of the groups are paraphyletic, i.e. have given rise to other groups. Quite a few authors working in the field use a classification based on purely on phylogeny, disregarding the anatomy and physiology. An example based on Janvier (1981, 1997), Shu et al.. (2003), and Benton (2004). is given here:


 * Subphylum Vertebrata
 * (Unranked group) Hyperoartia (lampreys)
 * Class †Conodonta
 * Class †Pteraspidomorphi
 * Class †Thelodonti
 * Class †Anaspida
 * Class †Galeaspida
 * Class †Pituriaspida
 * Class †Osteostraci
 * Infraphylum Gnathostomata (jawed vertebrates)
 * Class †Placodermi (Paleozoic armoured forms)
 * Class Chondrichthyes (cartilaginous fish)
 * Class †Acanthodii (Paleozoic "spiny sharks")
 * Superclass Osteichthyes (bony fish)
 * Class Actinopterygii (ray-finned fish)
 * Class Sarcopterygii (lobe-finned fish)
 * Subclass Coelacanthimorpha (coelacanths)
 * Subclass Dipnoi (lungfish)
 * Subclass Tetrapodomorpha (ancestral to tetrapods)
 * Superclass Tetrapoda (four-limbed vertebrates)
 * Class Amphibia (amphibians)
 * Series Amniota (amniotic embryo)
 * Class Sauropsida (reptiles and birds)
 * Class Aves (birds)
 * Class Synapsida (mammal-like reptiles)
 * Class Mammalia (mammals)

Etymology
The word vertebrate derives from Latin vertebrātus (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.