Xenacanthida (or Xenacanthiforms) is a super-order of extinct
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 ...
-like
elasmobranchs
Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including sharks (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of gil ...
that appeared during the Lower
Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
period. The order includes the families Xenacanthidae, Sphenacanthidae, Diplodoselachidae, and Orthacanthidae. The most notable members of the group are the genera ''
Xenacanthus
''Xenacanthus'' (from Ancient Greek ξένος, xénos, 'foreign, alien' + ἄκανθος, akanthos, 'spine') is a genus of prehistoric sharks. The first species of the genus lived in the later Devonian period, and they survived until the end o ...
'' and ''
Orthacanthus
''Orthacanthus'' is an extinct genus of fresh-water xenacanthid sharks, named by Louis Agassiz in 1836, ranging from the Upper Carboniferous until the Lower Permian. ''Orthacanthus'' had a nektobenthic life habitat, with a carnivorous diet. Mul ...
''. Some xenacanths may have grown to lengths of .Beck, Kimberley G.; oler-Gijón, Rodrigo; Carlucci, Jesse R.; Willis, Ray E. (December 2014). "Morphology and Histology of Dorsal Spines of the Xenacanthid Shark Orthacanthus platypternus from the Lower Permian of Texas, USA: Palaeobiological and Palaeoenvironmental Implications". ''Acta Palaeontologica Polonica''. 61 (1): 97–117. doi:10.4202/app.00126.2014 Most forms had large serrated spines extending backwards from the neck. ''Xenacanthus'' had characteristic teeth. Most xenacanths died out at the end of the
Permian
The Permian ( ) is a geologic period and 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 251.9 Mya. It is the last period of the Paleoz ...
in the
Permian Mass Extinction
The Permian ( ) is a geologic period and 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 251.9 Mya. It is the last period of the Paleoz ...
, with only a few forms surviving into 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 ...
period. They were native to freshwater, marginal marine and shallow marine habitats.
Description
The foundation of the tooth is prolonged lingually with a circlet button and a basal tubercle on the oral and aboral surfaces individually. Xenacanthida's teeth are famed by articulated bones, cephalic vertebrae and isolated teeth and found global in each aquatic and clean environment. The family Xenacanthidae consist of five genera that are ''
Xenacanthus
''Xenacanthus'' (from Ancient Greek ξένος, xénos, 'foreign, alien' + ἄκανθος, akanthos, 'spine') is a genus of prehistoric sharks. The first species of the genus lived in the later Devonian period, and they survived until the end o ...
'', ''
Triodus
''Triodus'' is an extinct genus of xenacanthidan shark that lived from the Carboniferous to the Triassic. It was a freshwater shark, and fossils have been found in the Chinle Formation and Black Prince Limestone of Arizona, the Petrified Fore ...
'', ''
Plicatodus
''Plicatodus'' is a prehistoric shark 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 ...
'', '' Mooreodontus'' and '' Wurdigneria''; all of these are distinguished by cross sections of the points, crown center, length of the median edge, type of vertical cristae, and microscopic anatomy. These kinds of fishes are largely marked from Paleozoic remains and their diversity cut drastically throughout the period of their extinction.
Xenacanths are divided into two groups based on dental characteristics. Group one has tricuspid crown containing two stout, slightly diverging lateral cusps pointing in the same direction, a high median cusp, with a crown-base angle almost at 90 degrees, a large, rounded, apical button with several foramina and multiple, 8-9 coarse vertical cristae on all the cusps. Group two has bicuspid crowns with two upright, asymmetric cusps, where the medial cusp is thicker than the distal one, and consistently lacks a median cusp.
Xenacanths had long dorsal fins, as well as a large spine projecting from the top of the head, which was a modified dorsal-fin spine. The spine is usually thought to have acted as a defense against attackers.
Ecology
Based on isotope analysis of teeth, some xenacanths are likely to have lived permanently in freshwater environments. The diet of freshwater xenacanths is known to have included
temnospondyls
Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carbo ...
as well as palaeoniscid fish. In the Early Permian freshwater lakes of the Saar–Nahe Basin in southern Germany, large xenacanths are suggested to have acted as the apex predators of this ecosystem.
Barbclabornia
''Barbclabornia'' is an extinct genus of xenacanthid shark 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 ...
'' Johnson, 2003
*** Genus: ''
Bransonella
''Bransonella'' is an extinct genus of marine Xenacanth shark which lived during the Paleozoic era. It is known only from teeth which are easily distinguished from related genera by ornamentation on the cusp shaped like an inverted "V" and fin s ...
Diplodoselachidae
Diplodoselachidae is a family of extinct xenacanthid sharks that ranged from the Permian to mid 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 year ...
Lebachacanthus
''Lebachacanthus'' is a genus of extinct Xenacanthid shark known from the early Permian. During the late Paleozoic, xenacanths ruled the freshwater swamps and bogs of the world, preying on small amphibian-like animals.
Paleobiology
''Lebachaca ...
Orthacanthus
''Orthacanthus'' is an extinct genus of fresh-water xenacanthid sharks, named by Louis Agassiz in 1836, ranging from the Upper Carboniferous until the Lower Permian. ''Orthacanthus'' had a nektobenthic life habitat, with a carnivorous diet. Mul ...
'' Agassiz, 1843
** Family:
Xenacanthidae
Xenacanthidae is a family of prehistoric sharks in the order Xenacanthida
Xenacanthida (or Xenacanthiforms) is a super-order of extinct shark-like elasmobranchs that appeared during the Lower Carboniferous period. The order includes the fami ...
Fritsch, 1889
*** Genus: '' Mooreodontus'' Ginter et al., 2010 North America, Europe, South America, Australia, India, Triassic
*** Genus: ''
Plicatodus
''Plicatodus'' is a prehistoric shark 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 ...
'' Hampe, 1995
*** Genus: ''
Triodus
''Triodus'' is an extinct genus of xenacanthidan shark that lived from the Carboniferous to the Triassic. It was a freshwater shark, and fossils have been found in the Chinle Formation and Black Prince Limestone of Arizona, the Petrified Fore ...
'' Jordan, 1849
*** Genus: ''
Xenacanthus
''Xenacanthus'' (from Ancient Greek ξένος, xénos, 'foreign, alien' + ἄκανθος, akanthos, 'spine') is a genus of prehistoric sharks. The first species of the genus lived in the later Devonian period, and they survived until the end o ...