''Xenacanthus'' (from Ancient Greek ξένος, xénos, 'foreign, alien' + ἄκανθος, akanthos, 'spine') is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of prehistoric

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s. The first species of the genus lived in the later

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...

period, and they survived until the end of the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...

, 202 million years ago. Fossils of various species have been found worldwide.

Description

''Xenacanthus'' had a number of features that distinguished it from modern sharks. This

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...

shark was usually about one meter (3.3 feet) in length, and never longer than . The

dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through c ...

was ribbonlike and ran the entire length of the back and round the tail, where it joined with the

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as see ...

. This arrangement resembles that of modern conger eels, and ''Xenacanthus'' probably swam in a similar manner. A distinctive spine projected from the back of the head and gave the genus its name. The spike has even been speculated to have been venomous, perhaps in a similar manner to a sting ray. This is quite plausible as the rays are close relatives to the sharks. The teeth had an unusual "V" shape, and it probably fed on small

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...

s and heavily scaled palaeoniscid fishes. As with all fossil sharks, ''Xenacanthus'' is mainly known because of fossilised teeth and spines.

Species

* ''X. texensis'' * ''X. atriossis'' * ''X. compressus'' * ''X. decheni'' * ''X. denticulatus'' * ''X. erectus'' * ''X. gibbosus'' * ''X. gracilis'' * ''X. howsei'' * ''X. laevissimus'' * ''X. latus'' * ''X. luedernesis'' * ''X. moorei'' - found in the lower levels of the

Chinle Formation The Chinle Formation is an Upper Triassic continental geological formation of fluvial, lacustrine, and palustrine to eolian deposits spread across the U.S. states of Nevada, Utah, northern Arizona, western New Mexico, and western Colorado. In N ...

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe, New Mexico, Santa Fe , LargestCity = Albuquerque, New Mexico, Albuquerque , LargestMetro = Albuquerque metropolitan area, Tiguex , Offi ...

, USA * ''X. ossiani'' * ''X. ovalis'' * ''X. parallelus'' * ''X. parvidens'' * ''X. ragonhai'' -

Rio do Rasto Formation The Rio do Rasto Formation is a Late Permian sedimentary geological formation in the South Region of Brazil. The official name is Rio do Rasto, although in some publications it appears as ''Rio do Rastro''. Geography It is found mainly in the ...

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

* ''X. robustus'' * ''X. serratus'' * ''X. slaughteri'' * ''X. taylori''

References

External links

* * {{Taxonbar, from=Q647507 Prehistoric shark genera Prehistoric cartilaginous fish genera Devonian cartilaginous fish Carboniferous cartilaginous fish Permian cartilaginous fish Triassic cartilaginous fish Rhaetian extinctions Prehistoric fish of Asia Triassic fish of Europe Prehistoric fish of North America Prehistoric fish of South America Permian Brazil Fossils of Brazil Fossil taxa described in 1848