Hybodontiformes

Source: Wikipedia, the free encyclopedia.

Hybodontiformes
Temporal range: 360.7–66 
Ma
 Late Devonian to Late Cretaceous
Fossil and life restoration of Asteracanthus, from the Late Jurassic of Europe
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Infraclass:
Euselachii
Order: †Hybodontiformes
Patterson, 1966
Families

See text

Hybodontiformes, commonly called hybodonts, are an

Euselachii. They are distinguished from other chondrichthyans by their distinctive fin spines and cephalic spines present on the heads of males. An ecologically diverse group, they were abundant in marine and freshwater environments during the late Paleozoic and early Mesozoic, but were rare in open marine environments by the end of the Jurassic, having been largely replaced by modern sharks, though they were still common in freshwater and marginal marine habitats. They survived until the end of the Cretaceous
, before going extinct.

Etymology

The term hybodont comes from the Greek word ὕβος or ὑβός meaning hump or hump-backed and ὀδούς, ὀδοντ meaning tooth. This name was given based on their conical compressed teeth.

Taxonomic history

Hybodonts were first described in the nineteenth century based on isolated fossil teeth (

Xenacanthiformes.[3] Hybodonts are divided into a number of families, but the higher level taxonomy of hybodonts, especially Mesozoic taxa, is poorly resolved.[4]

Description

cartilaginous skeletons usually disintegrating upon death like other chondrichthyans, hybodonts are generally described and identified based on teeth and fin spine fossils, which are more likely to be preserved.[4] Rare partial or complete skeletons are known from areas of exceptional preservation.[6][4][7]

Restoration of Hybodus hauffianus showing sexual dimorphism with fin claspers and cephalic spines present in males but absent in females

Hybodonts are recognized as having teeth with a prominent cusp which is higher than lateral cusplets.[8] Hybodont teeth are often preserved as incomplete fossils because the base of the tooth is not well attached to the crown.[8] Hybodonts were initially divided into two groups based on their tooth shape.[9] One group had teeth with acuminate cusps that lacked a pulp cavity; these are called osteodont teeth. The other group had a different cusp arrangement and had a pulp cavity, these are called orthodont teeth.[10] For example, the hybodont species Heterophychodus steinmanni have osteodont teeth with vascular canals of dentine which are arranged vertically parallel to each other, also called ‘tubular dentine’.[11] The crowns of these osteodont teeth are covered with a single layer of enameloid. Hybodont teeth served a variety of functions depending on the species, including grinding, crushing (durophagy), tearing, clutching, and even cutting.[4]

Hybodonts are characterized by having two dorsal fins each preceded by a fin spine. The fin spine morphology is unique to each hybodont species. The fin spines are elongate and gently curved towards the rear, with the posterior part of the spine being covered in hooked denticles, typically in two parallel rows running along the length of the spine, sometimes with a ridge between them. Part of the front of the spines are often covered in a ribbed ornamentation, while in some other hybodonts this region is covered in rows of small bumps. The spines are mineralised, and primary composed of

dermal denticles.[18] Hybodonts laid egg cases, similar to those produced by living cartilaginous fish. Most hybodont egg cases are assigned to the genus Palaeoxyris, which tapers towards both ends, with one end having a tendril which attached to substrate, with the middle section being composed of at least three twisted bands.[19]

  • Teeth of Durnovariaodus, a member of the family Hybodontidae
    Teeth of
    Hybodontidae
  • Jaw fragment of Strophodus a specialised durophagous hybodont
    Jaw fragment of Strophodus a specialised durophagous hybodont
  • Jaws of Asteracanthus, showing the arrangement of the teeth in jaws. The teeth were designed for grasping
    Jaws of Asteracanthus, showing the arrangement of the teeth in jaws. The teeth were designed for grasping
  • Preserved dentition of the acrodontid Acrodus, which had low, rounded teeth used in durophagy
    Preserved dentition of the acrodontid Acrodus, which had low, rounded teeth used in durophagy
  • Teeth of Planohybodus, a hybodontid, whos teeth were designed for tearing
    Teeth of Planohybodus, a hybodontid, whos teeth were designed for tearing

Ecology

estuarine
environment

Hybodont fossils are found in depositional environments ranging from marine to

fluvial (river deposits).[20] Many hybodonts are thought to have been euryhaline, able to tolerate a wide range of salinities.[21] Hybodonts inhabited freshwater environments from early in their evolutionary history, spanning from the Carboniferous onwards.[22] Based on isotopic analysis, some species of hybodonts are likely to have permanently lived in freshwater environments,[23][24] while others may have migrated between marine and freshwater environments.[25] One genus of hybodont, Onychoselache of the lower Carboniferous of Scotland, is suggested to have been capable of amphibious locomotion, similar to modern orectolobiform sharks such as bamboo and epaulette sharks, due to its well-developed pectoral fins.[16] It has been suggested that male hybodonts used their cephalic spines to grip females during mating.[26] Preserved egg cases of hybodonts assigned to Palaeoxyris indicate that at least some hybodonts laid their eggs in freshwater and brackish environments, with the eggs being attached to vegetation via a tendril. Laying of eggs in freshwater is not known in any living cartilaginous fish.[27][19][28] At least some hybodonts are suggested to have utlilized specific sites as nurseries, such as in the Triassic lake deposits of the Madygen Formation of Kyrgyzstan, where eggs of Lonchidion are suggested to have been laid on the lakeshore or upriver areas, where the juveniles hatched and matured, before migrating deeper into the lake as adults.[27]

Life restoration of Strophodus rebecae with other contemporary organisms from the Early Cretaceous (Valanginian-Hauterivian) Rosa Blanca Formation of Colombia

Some hybodonts like Hybodus are thought to have been active predators capable of feeding on swiftly moving prey,[2] with preserved stomach contents of a specimen of Hybodus hauffianus indicating that they fed on belemnites.[29] Hybodonts have a wide variety of tooth shapes. This variety suggests that they took advantage of multiple food sources.[8] It is thought that some hybodonts which had wider, flatter, teeth specialized in crushing or grinding hard-shelled prey (durophagy).[20] Often multiple species of hybodonts with different prey preferences coexisted within the same ecosystem.[30][11]

Evolutionary history

Fossil of Hamiltonichthys a primitive hybodont from the Carboniferous of North America

The earliest hybodont remains are from the latest

Cretaceous-Paleogene extinction event.[37]

Families and genera

The taxonomy of hybodonts is considered poorly resolved,[4] so the classification presented should not be taken as authoritative.

References

  1. ^ Zittel, K. von, 1911, Grunzuege der Palaontologie, 2 ed. II. Abt. Vertebrata, vii + 598 pp. R. Oldenburg Verlag, Muchen, Berlin.
  2. ^ a b Maisey, J. G., 2012, What is an ‘elasmobranch’? The impact of palaeontology in understanding elasmobranch phylogeny and evolution: Journal of Fish Biology, v. 80, no. 5, p. 918-951.
  3. S2CID 135297534
    .
  4. ^
    S2CID 234204226
    .
  5. doi:10.1016/j.cretres.2013.06.002
    .
  6. ^ Lane, J. A., and Maisey, J. G., 2009, Pectoral Anatomy of Tribodus limae (Elasmobranchii: Hybodontiformes) from the Lower Cretaceous of Northeastern Brazil: Journal of Vertebrate Paleontology, v. 29, no. 1, p. 25-38.
  7. doi:10.1111/j.1463-6395.2008.00382.x
    .
  8. ^ a b c Koot, M. B., Cuny, G., Tintori, A., and Twitchett, R. J., 2013, A new diverse shark fauna from the Wordian (Middle Permian) Khuff Formation in the interior Haushi-Huqf area, Sultanate of Oman: Palaeontology, v. 56, no. 2, p. 303-343.
  9. ^ Agassiz, L., 1833-1844, Recherches sur les poisons fossils. Neuchatel, 5 vols. 1420 pp. with supplement.
  10. ^ a b c d Maisey, J. G., 1982, The anatomy and interrelationships of Mesozoic hybodont sharks: American Museum Novitates, v. 2724.
  11. ^ a b Cuny, G., Suteethorn, V., Buffetaut, E., and Philippe, M., 2003, Hybodont sharks from the Mesozoic Khorat Group of Thailand: Mahasarakham University Journal, v. 22.
  12. ^ Maisey, J. G., 1978, Growth and form of spines in hybodont sharks: Palaeontology, v. 21, no. 3, p. 657-666.
  13. S2CID 86408226
    .
  14. S2CID 130984933
    .
  15. PMID 34026354
    .
  16. ^
    S2CID 140556654
    .
  17. S2CID 205411092
    .
  18. S2CID 88591830
    .
  19. ^
    ISSN 0077-7749
    .
  20. ^ a b c d e Rees, J. A. N., and Underwood, C. J., 2008, Hybodont sharks of the English Bathonian and Callovian (Middle Jurassic): Palaeontology, v. 51, no. 1, p. 117-147.
  21. doi:10.1016/j.pgeola.2013.05.001
    .
  22. S2CID 252136919
    .
  23. ISSN 0009-2541
    .
  24. S2CID 129236098
    .
  25. S2CID 129044300
    .
  26. S2CID 202904040
    .
  27. ^ a b Fischer, J. A. N., Voigt, S., Schneider, J. W., Buchwitz, M., and Voigt, S., 2011, A selachian freshwater fauna from the Triassic of Kyrgyzstan and its implication for Mesozoic shark nurseries: Journal of Vertebrate Paleontology, v. 31, no. 5, p. 937-953.
  28. ISSN 1103-5897
    .
  29. PMID 34721282
    .
  30. ^ Cappetta, H., Buffetaut, E., Cuny, G., and Suteethorn, V., 2006, A new Elasmobranch assemblage from the Lower Cretaceous of Thailand Palaeontology, v. 49, no. 3, p. 547-555.
  31. ^ Hairapetian, V. and Ginter, M. 2009. Famennian chondrichthyan remains from the Chahriseh section, central Iran. Acta Geologica Polonica, 59, 173–200.
  32. ^ a b c Hodnett, J-P., Elliott, D. K., and Olson, T. J. 2013. A new basal hybodont (Chondrichthyes, Hybodontiformes) from the Middle Permian (Roadian) Kaibab Formation, of northern Arizona. New Mexico Museum of Natural History and Science Bulletin, 60:103–108.
  33. ^ Coates, M. I., and Gess, R. W., 2007, A new reconstruction of Onychoselache Traquairi, comments on early Chondrichthyan pectoral girdles and hybodontiform phylogeny: Palaeontology, v. 50, no. 6, p. 1421-1446.
  34. doi:10.1016/j.jafrearsci.2021.104325
    .
  35. doi:10.1016/j.pgeola.2010.04.003
    .
  36. doi:10.1016/j.palaeo.2023.111471
    .
  37. ^
    doi:10.1016/j.palaeo.2004.02.049
    .
  38. ISSN 0022-3360
    .
  39. ISSN 1424-2818
    .
  40. S2CID 132467956
    .
Retrieved from "https://en.wikipedia.org/w/index.php?title=Hybodontiformes&oldid=1208941158"