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Evolutionary history

The first vertebrates

Vertebrates originated about 525 million years ago during the Cambrian explosion, which was an event of massive rise in organism diversity that occurred in the Cambrian period. The earliest known vertebrate is believed to be the Myllokunmingia. Another early vertebrate is Haikouichthys ercaicunensis, also from the Chengjiang fauna 518 million years ago. Unlike the other fauna that dominated the Cambrian, they had the basic vertebrate body plan; a notochord, rudimentary vertebrae, and a well-defined head and tail. All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed.

Molecular analysis since 1999 have suggested that the hagfishes are most closely related to lampreys, and so also are vertebrates. Others consider them a sister group of vertebrates in the common taxon of Craniata.

From fish to amphibians

The first jawed vertebrates appeared in the Ordovician and 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 jawless fishes, save for lampreys and hagfish, as well as the Placodermi, a group of armoured fish that dominated much of the late Silurian. The Devonian also saw the rise of the first labyrinthodonts, which was a transitional between fish and amphibians.

Mesozoic vertebrates

The reptiles appeared from labyrinthodont stock in the subsequent Carboniferous period. The anapsid and synapsid reptiles were common during the late Paleozoic, while the diapsids became dominant during the Mesozoic. In the sea, the bony fishes became dominant. The dinosaurs gave rise to the birds in the Jurassic. The demise of the dinosaurs at the end of the Cretaceous promoted expansion of the mammals, which had evolved from the therapsids, a group of synapsid reptiles, during the late Triassic Period.

After the dinosaurs

The post-dinosaur world have seen great diversification of bony fishes, frogs, birds and mammals.

Over half of all living vertebrate species (about 32,000 species) are fishes (non-tetrapod craniates), a diverse set of lineages that inhabit all the world’s aquatic ecosystems, from snow minnows (Cypriniformes) in Himalayan lakes at elevations over 4,600 meters (15,000 feet) to flatfishes (order Pleuronectiformes) in the Challenger Deep, the deepest ocean trench at about 11,000 meters (36,000 feet). Fishes of myriad varieties are the main predators in most of the world’s water bodies, both freshwater and marine. The rest of the vertebrate species are tetrapods, a single lineage that includes amphibians (frogs, with more than 5,800 species; salamanders, with about 580 species; and caecilians, with about 175 species); mammals (with over 5,400 species); and reptiles and birds (with more than 18,000 species). Tetrapods dominate the megafauna of most terrestrial environments (including fossorial and arboreal realms) and also include many partially or fully aquatic groups (e.g., sea snakes, penguins, cetaceans).