''Xenacanthus'' (from Ancient Greek

ξένος (; ) is a word used in the Greek language from Homer onwards. The most standard definition is 'stranger'. However, the word itself can be interpreted to mean different things based upon context, author and period of writing/speaking, signifying ...

, xénos, 'foreign, alien' + ἄκανθος, akanthos, 'spine') is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of xenacanth

cartilaginous fish 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 skeleto ...

. It lived in

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

environments, and fossils of various species have been found worldwide.

Description

''Xenacanthus'' is a relatively small member of its order. ''X. decheni'' reached about , ''X. meisenheimensis'' reached up to , ''X''. (''

Expleuracanthus ''Xenacanthus'' (from Ancient Greek wikt:ξένος, ξένος, xénos, 'foreign, alien' + wikt:ἄκανθος, ἄκανθος, akanthos, 'spine') is an extinct genus of Xenacanthida, xenacanth cartilaginous fish. It lived in freshwater environ ...

'') ''gaudryi'' reached . ''X. parallelus'' is one of the smallest xenacanths, males reached and females reached , both are fully grown. The

dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...

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

anal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported o ...

. 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. The teeth had an unusual "V" shape, and it probably fed on small

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s and heavily scaled palaeoniscid fishes. As with many xencanths, ''Xenacanthus'' is mainly known because of fossilised teeth and spines. Fossils are known from the

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

of North America, Europe, and South America.

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

species have been moved into the separate genus '' Mooreodontus''.

Species

* ''X. texensis'' * ''X. atriossis'' * ''X. compressus'' * ''X. indicus'' * ''X. decheni'' * ''X. denticulatus'' * ''X. erectus'' * ''X. gibbosus'' * ''X. gracilis'' * ''X. howsei'' * ''X. laevissimus'' * ''X. latus'' * ''X. luedernesis'' * ''X. ossiani'' * ''X. ovalis'' * ''X. parallelus'' * ''X. ragonhai'' - Rio do Rasto Formation,

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

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

References

External links

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