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.
Gnathostomata are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living gnathostomes have true teeth, paired appendages, the elastomeric protein of elastin, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin sheaths of neurons, and an adaptive immune system that has the discrete lymphoid organs of spleen and thymus, and uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.
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.
Teleostomi is an obsolete taxon of jawed vertebrates that supposedly includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives. The teleostomes include all jawed vertebrates except the chondrichthyans and the extinct class Placodermi.
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.
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.
Symmoriiformes is an extinct order of stem-group holocephalians. Originally named Symmoriida by Zangerl (1981), it has subsequently been known by several other names. Lund (1986) synonymized the group with Cladodontida, while Maisey (2008) corrected the name to Symmoriiformes in order to prevent it from being mistaken for a family. The symmoriiform fossils record begins during the late Devonian. Most of them died out at the start of the Permian, but Dwykaselachus is known from the Artinskian-Kungurian of South Africa. Teeth described from the Valanginian of France and Austria indicate that members of the family Falcatidae might have survived until the Early Cretaceous; however, these teeth were also argued to be more likely neoselachian teeth.
Symmoriidae is an extinct family of cartilaginous fish belonging to the order Symmoriiformes. Merbers of the family are known from the Devonian and Carboniferous periods.
Falcatus is an extinct genus of falcatid chondrichthyan which lived during the early Carboniferous Period in Bear Gulch bay in what is now Montana.
Branchial arches, or gill arches, are a series of paired bony "loops" that support the gills in fish. As gills are the primitive feature of vertebrates, all vertebrate embryos develop pharyngeal arches, though the eventual fate of these arches varies between taxa. In jawed fish, the first arch pair develops into the jaw. The second gill arches develop into the hyomandibular complex, which supports the back of the jaw and the front of the gill series. The remaining posterior arches support the gills. In amphibians and reptiles, many pharyngeal arch elements are lost, including the gill arches, resulting in only the oral jaws and a hyoid apparatus remaining. In mammals and birds, the hyoid is simplified further.
Symmorium is a dubious genus of extinct stethacanthid cartilaginous fish from the Devonian and Carboniferous of the United States (Illinois) and Russia. The type species, Symmorium reniforme, was named by Edward Drinker Cope in 1893 and several other species were originally classed under this genus, but they have since been classified into other genera such as Petalodus. Symmorium bears close similarity in size and appearance to Stethacanthus but the former is missing the "spine and brush" on its back. Some paleontologists think that the two forms are simply the males and females of related species, while other scientists think they were distinct genera.
Falcatidae is a family of Paleozoic cartilaginous fish belonging to the order Symmoriiformes. Members of this family include Falcatus, a small fish from the Bear Gulch Limestone of Montana. The family first appeared around the start of the Carboniferous, and there is some evidence that they survived well into the early Cretaceous, though its putative Cretaceous members were also argued to be more likely neoselachians.
The Fayetteville Shale is a geologic formation of Mississippian age composed of tight shale within the Arkoma Basin of Arkansas and Oklahoma. It is named for the city of Fayetteville, Arkansas, and requires hydraulic fracturing to release the natural gas contained within.
Kawichthys was an extinct genus of symmoriiform cartilaginous fish from Upper Pennsylvanian deposits of Kansas, United States. Kawichthys is known from two well preserved three-dimensional neurocrania: the holotype KUVP 152144 is associated with some disturbed and broken postcranial elements, but the braincase is partially crushed, and the paratype KUVP 56340. It was collected from the Douglas Group, between the Haskell Limestone and the lower beds of the overlying Robbins Shale, previously classified as members of the Lawrence Formation or Stranger Formation, but now recognized as members of the extension of the Cass Limestone classification into Kansas. It was first named by Alan Pradel, Paul Tafforeau, John G. Maisey and Philippe Janvier in 2011 and the type species is Kawichthys moodiei.
Gogoselachus is an extinct genus of cartilaginous fish known from the late Devonian of Australia. It is one of the earliest well-preserved Devonian chondrichthyans, as much more of the fish than just teeth and scales were preserved. This rare preservation reveals some unique discoveries about the evolution of the cartilage that was inside later cartilaginous fish such as sharks, rays, and chimaeras.
Dwykaselachus is an extinct genus of symmoriiform, a cartilaginous fish that lived in what is now South Africa during the Permian period around 280 million years ago. It was first discovered in the 1980s, in a nodule of sediments from the Karoo Supergroup. Dwykaselachus was named based on Dwyka Group, the group of sedimentary geological formation in the southeastern part of Africa. It represents the place where the type species Dwykaselachus oosthuizeni was found.
Cladodoides is a genus of extinct cartilaginous fish. It appeared in the Frasnian age of the late Devonian and possibly existed in the Tournaisian age of the early Carboniferous.
Ferromirum is an extinct genus of symmoriiform cartilaginous fish known from the late Devonian Ibâouane Formation in the southeastern Anti-Atlas of Morocco, with a single species Ferromirum oukherbouchi. It is known from a single well preserved skeleton, which is of a small individual less than half a metre in length. The jaws and hyoid arch are preserved uncrushed. The skull has large orbits which have sclerotic rings. The teeth are small and have a cladodont morphology. The body is slender. The first dorsal fin has a smooth fin spine, which curves posteriorly towards its tip.
Cosmoselachus is an extinct genus of symmoriiform chondrichthyan from the Upper Carboniferous aged Fayetteville Shale of Arkansas, United States. The genus contains a single species, C. mehlingi, which is known from a partial specimen that includes the cranium, jaws, gill arches, pectoral fins, and teeth.
