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Placoderms were among the first jawed fish (their jaws likely evolved from the first pair of gill arches), as well as the first vertebrates to have true teeth. They were also the first fish clade to develop pelvic fins , the second set of paired fins and the homologous precursor to hindlimbs in tetrapods .
The study of prehistoric fish is called paleoichthyology. A few living forms, such as the coelacanth are also referred to as prehistoric fish, or even living fossils, due to their current rarity and similarity to extinct forms. Fish which have become recently extinct are not usually referred to as prehistoric fish.
The order Arthrodira belongs to the class Placodermi, the large group of extinct prehistoric armored fish that is thought to have diverged over 400 million years ago from all sharks and bony fishes (and thus also all subsequent tetrapods, including mammals, birds, reptiles and amphibians).
Dunkleosteus is an extinct genus of large arthrodire ("jointed-neck") fish that existed during the Late Devonian period, about 382–358 million years ago.It was a pelagic fish inhabiting open waters, and one of the first vertebrate apex predators of any ecosystem.
There is a layer of enamel and even a layer of pulp. The whole shield is made up of thousands of small teeth fused together. This bony skull--one of the earliest in the fossil record--is made entirely of little teeth. Teeth originally arose to bite creatures (see Conodonts); later a version of teeth was used in a new way to protect them." [4]
Living Fossil: the Story of the Coelacanth. W. W. Norton. Sepkoski, Jack (2002). "A compendium of fossil marine animal genera". Bulletins of American Paleontology. 364: 560. Archived from the original on 20 February 2009; Weinberg, Samantha (1999). A Fish Caught in Time: The Search for the Coelacanth. Fourth Estate. Bruton, Mike (2015).
Helicoprion bessonovi, teeth at the front of the lower jaw (reversed for more natural position) Restoration of Romerodus (Caseodontidae). The Eugeneodontiformes, (also called Eugeneodontida) is an extinct and poorly known order of cartilaginous fishes.
Leedsichthys, a giant Jurassic pachycormid. This list of prehistoric bony fish is an attempt to create a comprehensive listing of all genera from the fossil record that have ever been considered to be bony fish (class Osteichthyes), excluding purely vernacular terms.