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.
Acanthodii or acanthodians is an extinct class of gnathostomes. They are currently considered to represent a paraphyletic grade of various fish lineages basal to extant Chondrichthyes, which includes living sharks, rays, and chimaeras. Acanthodians possess a mosaic of features shared with both osteichthyans and chondrichthyans. In general body shape, they were similar to modern sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteians.
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.
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.
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.
Stereosternum tumidum is an extinct genus of mesosaur marine reptile from the Early Permian of Brazil and also the Great Karoo Basin of South Africa. The taxon mesosaur is a monophyletic group containing Brazilosaurus sanpauloensis and Mesosaurus tenuidens.
Triodus is an extinct genus of xenacanthiform cartilaginous fish that lived from the Carboniferous to the Permian. In 2017, a new species Triodus richterae was described from the Rio do Rasto Formation of Brazil.
Sphenacanthus is an extinct genus of a chondrichtyan xenacanthiform that belongs to the Sphenacanthidae family and lived from the Late Devonian, through Carboniferous until the Late Permian period in Scotland, Spain, Russia and Brazil. It lived 359 million years ago, and probably it was one of the first member of the elasmobranchians, the lineage that leads to the modern sharks. Sphenacanthus probably hunts small fishes and, unlike their modern-day relatives, its inhabited fresh water lagoons. Sphenacanthus had seven fins, two in the upper part and five in the underside, and it had a heterodont dentition and mandibles relatively long and deeper. Sphenacanthus serrulatus is still only known from incomplete neurocranial remains and associated dermal material. These suggest that it was a relatively large shark, probably well over one meter in length when fully grown. Its body form was probably similar to that of other phalacanthous sharks.
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.
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.
The Archer City Formation is a geological formation in north-central Texas, preserving fossils from the Asselian and early Sakmarian stages of the Permian period. It is the earliest component of the Texas red beds, introducing a tropical ecosystem which will persist in the area through the rest of the Early Permian. The Archer City Formation is preceded by the cool Carboniferous swamp sediments of the Markley Formation, and succeeded by the equally fossiliferous red beds of the Nocona Formation. The Archer City Formation was not named as a unique geological unit until the late 1980s. Older studies generally labelled its outcrops as the Moran or Putnam formations, which are age-equivalent marine units to the southwest.
The Arroyo Formation, sometimes termed the Lower Clear Fork Formation, is a geologic formation in Texas. It preserves fossils dating back to the Kungurian stage of the Permian period. It is the lower-most portion of the Clear Fork Group, part of a series of fossiliferous Permian strata in the south-central United States known as the red beds.
Antarctilamna is an extinct genus of Devonian cartilaginous fish originally exemplified by Antarctilamna prisca from South Eastern Australia and Antarctica. The latest occurring described species is Antarctilamna ultima from the Waterloo Farm lagerstätte in South Africa. Antarctilamna has robust ctenacanthid-like spines which lack a deep insertion area, and are borne in front of the first dorsal fin; in addition to distinctive diplodont teeth with small intermediate cusps. Antarctilamna-like spines, known from the Bunga Beds locality in Australia have been ascribed to A. prisca.
Barbclabornia is an extinct genus of xenacanth from the Early Permian and possibly upper Pennsylvanian of North America. The genus contains a single described species: B. luedersensis. It has been found in several places within Asselian and Atinskian formations, including the Clear Fork, Albany, Wichita, and Dunkard Groups. There are possible examples from the Gzhelian-aged Admire, Monongahela, and Conemaugh groups.
Ctenacanthiformes is an extinct order of cartilaginous fish. They possessed ornamented fin spines at the front of their dorsal fins and cladodont-type dentition, that is typically of a grasping morphology, though some taxa developed cutting and gouging tooth morphologies. Some ctenacanths are thought to have reached sizes comparable to the great white shark, with body lengths of up to 7 metres (23 ft) and weights of 1,500–2,500 kilograms (3,300–5,500 lb), while others reached lengths of only 30 centimetres (12 in). The earliest ctenacanths appeared during the Frasnian stage of the Late Devonian, with the group reaching their greatest diversity during the Early Carboniferous (Mississippian), and continued to exist into at least the Middle Permian (Guadalupian). Some authors have suggested members of the family Ctenacanthidae may have survived into the Cretaceous based on teeth found in deep water deposits of Valanginian age in France and Austria, however, other authors contend that the similarity of these teeth to Paleozoic ctenacanths is only superficial, and they likely belong to neoselachians instead.
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 subperiod of the Carboniferous period. A single species is known, Dracopristis hoffmanorum, which is named in honor of Ralph and Jeanette Hoffman.
Lebachacanthus is a genus of extinct xenacanth cartilaginous fish known from the late Carboniferous-Early Permian of Europe. Well-preserved specimens, originally identified as Orthacanthus, are known from Meisenheim Formation in Germany. During the late Paleozoic, xenacanths were the apex predators of freshwater ecosystems, preying on small amphibians.
