Chondrichthyes is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or bony fish, which have skeletons primarily composed of bone tissue. Chondrichthyes are aquatic vertebrates with paired fins, paired nares, placoid scales, conus arteriosus in the heart, and a lack of opercula and swim bladders. Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.
Elasmobranchii is a subclass of Chondrichthyes or cartilaginous fish, including modern sharks, rays, skates, and sawfish. Members of this subclass are characterised by having five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins and small placoid scales on the skin. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. The details of this jaw anatomy vary between species, and help distinguish the different elasmobranch clades. The pelvic fins in males are modified to create claspers for the transfer of sperm. There is no swim bladder; instead, these fish maintain buoyancy with large livers rich in oil.
Cladoselache is an extinct genus of shark-like chondrichthyan from the Late Devonian (Famennian) of North America. It was similar in body shape to modern lamnid sharks, but was not closely related to lamnids or to any other modern (selachian) shark. As an early chondrichthyan, it had yet to evolve traits of modern sharks such as accelerated tooth replacement, a loose jaw suspension, enameloid teeth, and possibly claspers.
Orthacanthus is an extinct genus of fresh-water xenacanthiform cartilaginous fish, named by Louis Agassiz in 1843, ranging from the Upper Carboniferous into the Lower Permian. Orthacanthus had a nektobenthic life habitat, with a carnivorous diet. Multiple authors have also discovered evidence of cannibalism in the diet of Orthacanthus and of "filial cannibalism" where adult Orthacanthus preyed upon juvenile Orthacanthus. Synonyms of the genus Orthacanthus are Dittodus Owen, 1867, Didymodus Cope, 1883, Diplodus Agassiz, 1843, Chilodus Giebel, 1848.
Helicoprion is an extinct genus of shark-like eugeneodont fish. Almost all fossil specimens are of spirally arranged clusters of the individuals' teeth, called "tooth whorls", which in life were embedded in the lower jaw. As with most extinct cartilaginous fish, the skeleton is mostly unknown. Fossils of Helicoprion are known from a 20 million year timespan during the Permian period from the Artinskian stage of the Cisuralian to the Roadian stage of the Guadalupian. The closest living relatives of Helicoprion are the chimaeras, though their relationship is very distant. The unusual tooth arrangement is thought to have been an adaption for feeding on soft bodied prey, and may have functioned as a deshelling mechanism for hard bodied cephalopods such as nautiloids and ammonoids. In 2013, systematic revision of Helicoprion via morphometric analysis of the tooth whorls found only H. davisii, H. bessonowi and H. ergassaminon to be valid, with some of the larger tooth whorls being outliers.
Xenacanthus is an extinct genus of xenacanth cartilaginous fish. It lived in freshwater environments, and fossils of various species have been found worldwide.
Stethacanthus is an extinct genus of shark-like cartilaginous fish which lived from the Late Devonian to Late Carboniferous epoch, dying out around 298.9 million years ago. Fossils have been found in Australia, Asia, Europe and North America.
Xenacanthida is an order or superorder of extinct shark-like chondrichthyans known from the Carboniferous to Triassic. They were native to freshwater, marginal marine and shallow marine habitats. Some xenacanths may have grown to lengths of 5 m (16 ft). Most xenacanths died out at the end of the Permian in the End-Permian Mass Extinction, with only a few forms surviving into the Triassic.
Edestus is an extinct genus of eugeneodontid holocephalian fish known from the Late Carboniferous (Pennsylvanian) of the United Kingdom, Russia, and the United States. Most remains consist of isolated curved blades or "whorls" that are studded with teeth, that in life were situated within the jaws. Edestus is a Greek name derived from the word edeste, in reference to the aberrant quality and size of the species' teeth. The largest species, E. heinrichi, has been conservatively estimated to reach greater than 6.7 m (22 ft) in length, around the size of the largest known great white shark, possibly making it the largest marine predator to have ever existed up to that point.
Stethacanthidae is an extinct family of prehistoric holocephalians. It is estimated to have existed approximately between 380 and 300 million years ago. Members of this family are noted for their peculiar dorsal fin.
Ptychodus is a genus of extinct large durophagous (shell-crushing) lamniform sharks from the Cretaceous period, spanning from the Albian to the Campanian. Fossils of Ptychodus teeth are found in many Late Cretaceous marine sediments worldwide. At least 16 species are considered valid, with the largest members of the genus suggested to have grown up to 10 meters long. The youngest remains date to around 85 million years ago. A large number of remains have been found in the former Western Interior Seaway.
Fadenia is an extinct genus of eugeneodontid holocephalian chondrichthyan from the Carboniferous Period of Missouri, the Permian period of Greenland, and the Early Triassic epoch of British Columbia, Canada.
Hybodontiformes, commonly called hybodonts, are an extinct group of shark-like cartilaginous fish (chondrichthyans) which existed from the late Devonian to the Late Cretaceous. Hybodonts share a close common ancestry with modern sharks and rays (Neoselachii) as part of the clade Euselachii. They are distinguished from other chondrichthyans by their distinctive fin spines and cephalic spines present on the heads of males. An ecologically diverse group, they were abundant in marine and freshwater environments during the late Paleozoic and early Mesozoic, but were rare in open marine environments by the end of the Jurassic, having been largely replaced by modern sharks, though they were still common in freshwater and marginal marine habitats. They survived until the end of the Cretaceous, before going extinct.
The Eugeneodontida, sometimes also called Eugeneodontiformes, is an extinct and poorly known order of cartilaginous fishes. They possessed "tooth-whorls" on the symphysis of either the lower or both jaws and pectoral fins supported by long radials. They probably lacked pelvic fins and anal fins. The palatoquadrate was either fused to the skull or reduced. Now determined to be within the Holocephali, their closest living relatives are ratfish. The eugeneodonts are named after paleontologist Eugene S. Richardson, Jr. The Eugeneodontida disappeared in the Early Triassic. The geologically youngest fossils of the group are known from the Sulphur Mountain Formation, Vardebukta Formation and Wordie Creek Formation (Greenland).
Plicatodus is a prehistoric cartilaginous fish in the family Xenacanthidae that lived in Europe during the late Carboniferous and Early Permian Periods. It was described by Oliver Hampe in 1995, and the type species is Plicatodus jordani. The type locality for this genus is the Saar-Nahe basin.
Toxoprion is an extinct genus of eugeneodont holocephalans whose fossils are found in marine strata from the Early Carboniferous until the Late Permian near Eureka, Nevada.
Bransonella is an extinct genus of marine Xenacanth which lived during the Paleozoic era. It is known only from teeth which are easily distinguished from related genera by ornamentation on the cusp shaped like an inverted "V" and fin spines Teeth attributed to this genus are small, no greater than 2 millimeters. This suggests a small body size likely not exceeding a meter in length. It has been suggested their lifestyle was similar to modern Catsharks. It was used to erect a new order along with Barbclabornia based on ornamentation.
Cretacladoides is a genus of chondrichthyan, possibly a falcatid, found in France and Austria. Known solely from teeth, mainly found in the Klausrieglerbach locality of Austria, it consists of two species, C. ogiveformis and C. noricum. Assuming a falcatid identity, it is the most recent member of the family, which otherwise became extinct at the end of the Carboniferous.
Dracopristis is an extinct genus of ctenacanth that lived around 307 million years ago, during the Pennsylvanian sub-period of the Carboniferous period. The fish had 12 rows of short, squat teeth, and an array of spines on its dorsal fins. The main differentiation between ctenacanthiformes and true sharks is that ctenacanthiform mouths are larger but less flexible than the true sharks. The spines of the holotype fossil are about 0.57 meters long, and the whole body was around 2 meters (6 ft) long.
Lebachacanthus is a genus of extinct xenacanth cartilaginous fish known from the late Carboniferous-Early Permian of Europe. During the late Paleozoic, xenacanths were the apex predators of freshwater ecosystems, preying on small amphibians.
