Xenacanthida (or Xenacanthiformes) is an order or superorder of extinct shark-like chondrichthyans (cartilaginous fish) 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 ...

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 ...

. They were native to freshwater, marginal marine and shallow marine habitats. Some xenacanths may have grown to lengths of . Most xenacanths died out at the end 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 ...

in the End-Permian Mass Extinction, with only a few forms surviving into the Triassic.

Description

The foundation of the tooth is prolonged lingually with a circlet button and a basal tubercle on the oral and aboral surfaces individually. The family Xenacanthidae consists of five genera: ''

Xenacanthus ''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 ...

'', '' Triodus'', '' Plicatodus'', '' 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. Xenacanths are divided into two groups based on dental characteristics. Group one has tricuspid crowns 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. The bodies of xenacanths are elongate and eel-like. 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. They also bore two anal fins, with the tail (caudal) fin being pseudo- diphycercal. They were probably slow swimmers that swam using side to side undulations of the body ( anguilliform locomotion). Some xenacanths like '' Barbclabornia'', are thought to have reached lengths of . While others such as ''Triodus'' were only around long.

Ecology

Many xenacanths are thought to have been euryhaline and to have migrated between freshwater and marine environments. ''Orthacanthus platypternus'' from the Early Permian of North America is suggested to have been catadromous, migrating into freshwater environments as a juvenile before returning to the sea as an adult. Based on isotope analysis of teeth, some xenacanths have been suggested to have lived permanently in freshwater environments. However, this proposal has been criticised by some authors, as the mineralization window of individual teeth only spans a short interval of time of days to weeks, and may not be reflective of long term behaviour. However isotopic analysis of fin spines of ''Orthacanthus'' and ''Triodus'' from the Early Permian of France, suggests that at least for these species, growth exclusively occurred in freshwater environments. Fossil egg cases assigned to the genus '' Fayolia'', which were probably produced by xenacanths, have a helically twisted collarette running around them, similar to the eggs of bullhead sharks, and taper towards both ends, with one end having a tendril. These eggs are typically found in freshwater deposits. A number of xenacanths are likely to have been fully marine, such as the small primitive genus '' Bransonella'', which is thought to have had a seafloor dwelling (

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

) ecology similar to that of a modern catshark. Most xenacanths are thought to have been ambush predators. The diet of freshwater xenacanths is known to have included

temnospondyl Temnospondyli (from Greek language, Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') or temnospondyls is a diverse ancient order (biology), order of small to giant tetrapods—often considered Labyrinth ...

amphibians as well as palaeoniscid fish, acanthodians, and other xenacanths. Large xenacanths are suggested to have acted as the

apex predator An apex predator, also known as a top predator or superpredator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the hig ...

s of late Paleozoic freshwater ecosystems, such as the Early Permian freshwater lakes of the Saar–Nahe Basin in southern Germany. The large ''Barbclabornia'', which has proportionally tiny teeth, has been suggested to be a

filter feeder Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a s ...

by some authors, different from the lifestyle inferred for other xenacanths.

Taxonomy

Xenacanths are typically placed as stem-group elasmobranchs, more closely related to modern

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s and rays than to

Holocephali Holocephali (Sometimes spelled Holocephala; Romanization of Greek, Greek for "complete head" in reference to the fusion of Palatoquadrate, upper jaw with the rest of the skull) is a Subclass (biology), subclass of Chondrichthyes, cartilaginous fi ...

, which includes chimaeras. A number of recent phylogenetic studies have found xenacanths to be nested within the traditional " Ctenacanthiformes", which would render that group

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

. Cladogram after Luccisano et al. 2021:

Subdivisions

* Order: Bransonelliformes Hampe & Ivanov, 2007 *** Genus: '' Barbclabornia'' Johnson, 2003 (Early Permian, possibly also Late Carboniferous, North America) *** Genus: '' Bransonella'' Harlton, 1933 (Early Carboniferous-Middle Permian, Worldwide) * Order: Xenacanthiformes Berg, 1955 ** Family: Diplodoselachidae Dick, 1981 *** Genus: '' Dicentrodus'' Traquair, 1888 (Early Carboniferous, Europe, North America) *** Genus: '' Diplodoselache'' Dick, 1981 (Early Carboniferous, Europe) *** Genus: '' Hagenoselache'' Hampe & Heidkte, 1997 (mid-Carboniferous, Europe) *** Genus: '' Hokomata'' Hodnett & Elliott, 2018 (mid-Carboniferous, North America) *** Genus: '' Lebachacanthus'' Soler-Gijon, 1997 (Late Carboniferous-Early Permian, Europe) *** Genus: '' Reginaselache'' Turner & Burrow, 2011 (Early Carboniferous, Australia) ** Family: Sphenacanthidae Maisey, 1982 *** Genus: '' Sphenacanthus'' Agassiz, 1837 (Early Carboniferous-Late Permian, Worldwide) *** Genus: '' Xenosynechodus'' Agassiz, 1980 (Middle-Late Permian, Europe, later authors have rejected its placement as a xenacanth) *** Genus: '' Desinia'' Ivanov, 2022 (Middle-Late Permian, Europe) ** Family: Orthacanthidae Heyler & Poplin 1990 *** Genus: '' Orthacanthus'' Agassiz, 1843 (Late Carboniferous-Early Permian, Europe, North America) ** Family: Xenacanthidae Fritsch, 1889 *** Genus: '' Mooreodontus'' Ginter et al., 2010 (Middle-Late Triassic, Worldwide) *** Genus: '' Plicatodus'' Hampe, 1995 (Late Carboniferous-Early Permian, Europe) *** Genus: '' Triodus'' Jordan, 1849 (Late Carboniferous-Middle Permian, Europe, North America, South America) *** Genus: ''

'' Beyrich, 1848 (Carboniferous-Permian, Worldwide) *** Genus: '' Wurdigneria'' Richter, 2005 (Middle-Late Permian, South America) ** ''incertae sedis'' *** Genus: '' Tikiodontus'' Bhat, Ray & Datta, 2018 (Late Triassic, India)

Description

Ecology

Taxonomy

Subdivisions

References

Further reading