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Gnathostomata are the jawed vertebrates. The term derives from Greek: γνάθος "jaw" + στόμα "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing jaws, living gnathostomes have teeth, paired appendages, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin sheaths of neurons. Another is an adaptive immune system that uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.
Placodermi is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches. Placoderms are thought to be paraphyletic, consisting of several distinct outgroups or sister taxa to all living jawed vertebrates, which originated among their ranks. This is illustrated by a 419-million-year-old fossil, Entelognathus, from China, which is the only known placoderm with a type of bony jaw like that found in modern bony fishes. This includes a dentary bone, which is found in humans and other tetrapods. The jaws in other placoderms were simplified and consisted of a single bone. Placoderms were also the first fish to develop pelvic fins, the precursor to hindlimbs in tetrapods, as well as true teeth. Paraphyletic groupings are problematic, as one can not talk precisely about their phylogenic relationships, characteristic traits, and complete extinction. 380-million-year-old fossils of three other genera, Incisoscutum, Materpiscis and Austroptyctodus, represent the oldest known examples of live birth. In contrast, one 2016 analysis concluded that placodermi are likely monophyletic.
Psarolepis is a genus of extinct lobe-finned fish which lived around 397 to 418 million years ago. Fossils of Psarolepis have been found mainly in South China and described by paleontologist Xiaobo Yu in 1998. It is not known certainly in which group Psarolepis belongs, but paleontologists agree that it probably is a basal genus and seems to be close to the common ancestor of lobe-finned and ray-finned fishes. In 2001, paleontologist John A. Long compared Psarolepis with Onychodontiform fishes and refer to their relationships.
Andreolepis is an extinct genus of prehistoric fish, which lived around 420 million years ago. It was described by Walter Gross in 1968 based on scales found in the Hemse Formation in Gotland, Sweden. It is placed in the monogeneric family Andreolepididae and is generally regarded as a primitive member of the class Actinopterygii based on its ganoid scale structure; however some new research regards it as a stem group of osteichthyans.
Plectocretacicus is an extinct genus of prehistoric ray-finned fish that lived during the lower Cenomanian. It contains a single species, P. clarae. Plectocretacicus is the earliest known member of the order Tetraodontiformes.
Dastilbe is an extinct genus of prehistoric bony fish from the Aptian.
Elopopsis is an extinct genus of prehistoric bony fish that lived from the Cenomanian to Campanian.
Chongichthys is an extinct genus of prehistoric bony fish that lived during the Oxfordian stage of the Late Jurassic epoch. Fossils of the genus have been found in the Quebrada El Profeta of Chile.
Lophar miocaenus is an extinct bony fish almost identical in form to the living bluefish, Pomatomus saltatrix, differing in its dentition, which consisted of "thick, conical subequal teeth" instead of the sharp, slender teeth and canines seen in bluefish. L. miocaenus lived during the Upper Miocene subepoch of Southern California.
Guiyu oneiros is the earliest articulated bony fish discovered. The generic name Guiyu is a transliteration of the Mandarin 鬼魚 guǐyú "ghost fish" and the specific name oneiros is from Greek ὄνειρος "dream". Fossils of Guiyu have been found in what is now Qujing, Yunnan, China, in a late Silurian marine strata, about 419 million years old. Guiyu remains have been found articulated, missing only the caudal fin. The living fish is estimated to have been 33 cm long.
Garnbergia is an extinct genus of prehistoric bony fish that lived during the Anisian stage of the Middle Triassic epoch.
Eosalmo is a genus of extinct salmon which lived during the Eocene epoch.
Luisichthys is an extinct genus of prehistoric bony fish that lived from the Oxfordian to the early Tithonian stage of the Late Jurassic epoch.
Ptycholepis is an extinct genus of prehistoric ray-finned fish having the head and opercular bones ornamented with ridges of ganoin, minute teeth, and thick scales.
Eutrichiurides is an extinct genus of prehistoric cutlass fish.
Graphiuricthys is an extinct genus of prehistoric bony fish.
Ligulalepis is an extinct genus of prehistoric bony fish. Ligulalepis was first described from isolated scales found in the Taemas-Wee jasper limestones of New South Wales by Dr Hans-Peter Schultze (1968) and further material described by Burrow (1994). A nearly complete skull found in the same general location was described in Nature by Basden et al. (2000) claiming the genus was closely related to basal ray-finned fishes (Actinopterygii). In 2015 Flinders University student Benedict King found a more complete new skull of this genus which was formally described by Clement et al. (2018), showing the fish to be on the stem of all osteichthyans.
Litoptychius is an extinct genus of prehistoric bony fish.
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.
Ohmdenia is an extinct genus of prehistoric bony fish that lived from the Toarcian stage of the Early Jurassic epoch. Ohmdenia was first described in 1953 by B. Hauff, based on a fossil found in the well-known Posidonia Shale in Holzmaden, Germany. For a long time this animal has been considered a close relative of Birgeria, a great predator typical of the Triassic period with an uncertain systematic position. Further studies have shown similarities with the pachicormiforms, a group of neopolitics considered close to the origin of teleosts and also including giant forms and planctives. Some studies have erroneously indicated Ohmdenia as a synonym of Saurostomus, other studies have instead placed Ohmdenia as an important evolutionary passage between the basal pachicormiforms and the more derived planktivore pachicormiformes.
References
- ↑ Sepkoski, Jack (2002). "A compendium of fossil marine animal genera". Bulletins of American Paleontology. 364: 560. Archived from the original on 2009-02-20. Retrieved 2009-02-27.
