Vertebrate

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.

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