''Xenacanthus'' (from Ancient Greek
ξένος
''Xenos'' (; ) 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, sig ...
, xénos, 'foreign, alien' +
ἄκανθος, akanthos, 'spine') is a
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
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 Selachimo ...
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, whe ...
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 period ...
, 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 include ...
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 conv ...
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 se ...
. This arrangement resembles that of modern
conger eel
''Conger'' ( ) is a genus of marine congrid eels. It includes some of the largest types of eels, ranging up to 2 m (6 ft) or more in length, in the case of the European conger. Large congers have often been observed by divers during t ...
s, 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 decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...
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 Ne ...
,
New Mexico
)
, population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano)
, seat = Santa Fe
, LargestCity = Albuquerque
, LargestMetro = Tiguex
, OfficialLang = None
, Languages = English, Spanish ( New Mexican), Navajo, Ker ...
,
USA
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
* ''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 B ...
,
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