A backbone and skull define some sixty-five thousand species
Vertebrates—also called craniates—wrap spinal cord and brain in vertebral column and skull. Subphylum Vertebrata holds about 65,000 species, the largest cut of phylum Chordata, spanning mammals, birds, amphibians, and fishes. The first appear in the Cambrian explosion some 518 million years ago.
Every chordate shares five hallmarks: a stiff rod called the notochord, a dorsal nerve tube, paired gill openings, a tail running past the anus, and an endostyle, the organ that became the vertebrate thyroid. Vertebrates add a long list of their own: a skull and spine of bone or cartilage, a brain in at least three parts, a muscular heart of two or more chambers, semicircular canals, camera eyes, a nose, and kidneys with glomeruli. Five molecular markers in proteins are shared by every vertebrate and by no other animal. The name comes via Latin vertebra, "joint," from vertere, "to turn."
Some 65,000 species make this the biggest ranked group among chordates, split roughly evenly between fishes and four-limbed animals, yet vertebrates still account for under five percent of described animals. They run from the frog Brachycephalus pulex, about 6.45 mm long as an adult, to the blue whale at up to 33 m and roughly 150 tonnes.
The story starts in China's Chengjiang biota about 518 million years ago, with jawless creatures such as Haikouichthys and Myllokunmingia that already had a head, a tail and rudimentary vertebrae. Jaws, which evolved from the first pair of gill arches, arrived in the late Ordovician or Silurian, and the Devonian became the "Age of Fishes." Lobe-finned fishes with lungs hauled themselves onto land, their bony fins becoming the legs of the first tetrapods late in the Devonian; amphibians and then egg-shielding amniotes followed in the Carboniferous. Mammals and dinosaurs appeared in the Triassic, and birds branched from dinosaurs in the Jurassic.
Classification took two centuries to settle. Lamarck defined vertebrates in 1801 but lumped lancelets and tunicates with molluscs, while Linnaeus had filed hagfish among worms. Balfour made Vertebrata a chordate subphylum in 1880–1881. Whether hagfish belong was argued until DNA work in 2002 and Philippe Janvier's 2010 verdict that lampreys and hagfish form one natural group. Since 1970 many vertebrate populations have fallen steeply under land clearing, overexploitation, climate change, pollution and invasive species.
Source: Vertebrate
