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Hybodontiformes

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

See text

Hybodontiformes,commonly calledhybodonts,are anextinctgroup of shark-likecartilaginous fish(chondrichthyans) which existed from the lateDevonianto theLate Cretaceous.Hybodonts share a close common ancestry with modernsharksandrays(Neoselachii) as part of the cladeEuselachii.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 latePaleozoicand earlyMesozoic,but were rare in open marine environments by the end of theJurassic,having been largely replaced by modern sharks, though they were still common in freshwater and marginal marine habitats. They survived until the end of theCretaceous,before going extinct.

Etymology

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The term hybodont comes from the Greek wordὕβοςorὑβόςmeaning hump or hump-backed andὀδούς, ὀδοντmeaning tooth. This name was given based on their conical compressed teeth.

Taxonomic history

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Hybodonts were first described in the nineteenth century based on isolated fossil teeth (Agassiz,1837). Hybodonts were first separated from livingsharksby Zittel (1911).[1]Although historically argued to have a close relationship with the modern shark orderHeterodontiformes,this has been refuted.[2]Hybodontiformes aretotal group-elasmobranchsand thesister grouptoNeoselachii,which includes modern sharks andrays.Hybodontiformes and Neoselachii are grouped together in the clade Euselachii, to the exclusion of other total-group elasmobranchs likeXenacanthiformes.[3]Hybodonts are divided into a number of families, but the higher level taxonomy of hybodonts, especiallyMesozoictaxa, is poorly resolved.[4]

Description

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Specimen of "Hybodus "fraasifrom the Late Jurassic of Germany,which some studies have included inEgertonodus

The largest hybodonts reached lengths of 2–3 metres (6.6–9.8 ft),[4]while some other hybodonts were much smaller, with adult body lengths of around 25 centimetres (0.82 ft).[5]Hybodonts had a generally robust bodyform. Due to theircartilaginousskeletons usually disintegrating upon death like otherchondrichthyans,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 ofexceptional preservation.[6][4][7]

Restoration ofHybodus hauffianusshowingsexual dimorphismwithfin claspersand 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 speciesHeterophychodus steinmannihave 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 ofenameloid.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 ofosteodentine,while the ornamentation is formed of enamel.[12]Similar fin spines are also found in many extinct chondrichthyan groups as well as in some modern sharks likeHeterodontusandsqualids.[13]Male hybodonts had either one or two pairs of cephalic spines on their heads, a characteristic distinctive to hybodonts.[14]These spines, while of variable placement,[10]were always placed posterior to the eye socket,[15]and were composed of a base divided into three lobes, with the main part of the spine being backwardly curved, most specimens of which had a barb near the apex.[10]These spines, like the fin spines, were mineralised, with the base composed of osteodentine,[16]while the main part of the spine was covered in enamel. Male hybodonts possessed finclaspersused in mating, like modern sharks.[10]Hybodonts had a fullyheterocercaltail fin, where the upper lobe of the fin was much larger than the lower one due to the spine extending into it.[17]Like living sharks and rays, the skin of hybodonts was covered withdermal denticles.[18]Hybodonts laidegg cases,similar to those produced by living cartilaginous fish. Most hybodont egg cases are assigned to the genusPalaeoxyris,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 ofDurnovariaodus,a member of the familyHybodontidae
  • Jaw fragment of Strophodus a specialised durophagous hybodont
    Jaw fragment ofStrophodusa specialised durophagous hybodont
  • Jaws of Asteracanthus, showing the arrangement of the teeth in jaws. The teeth were designed for grasping
    Jaws ofAsteracanthus,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 acrodontidAcrodus,which had low, rounded teeth used indurophagy
  • Teeth of Planohybodus, a hybodontid, whose teeth were designed for tearing
    Teeth ofPlanohybodus,a hybodontid, whose teeth were designed for tearing

Ecology

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Hybodont egg cases (Palaeoxyris) attached to aNeocalamitesstem in anestuarineenvironment

Hybodont fossils are found in depositional environments ranging from marine tofluvial(river deposits).[20]Many hybodonts are thought to have beeneuryhaline,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,Onychoselacheof the lowerCarboniferousof Scotland, is suggested to have been capable of amphibious locomotion, similar to modernorectolobiformsharks 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]Preservedegg casesof hybodonts assigned toPalaeoxyrisindicate 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 theMadygen Formationof Kyrgyzstan, where eggs ofLonchidionare 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 ofStrophodus rebecaewith other contemporary organisms from the Early Cretaceous (Valanginian-Hauterivian)Rosa Blanca Formationof Colombia

Some hybodonts likeHybodusare thought to have been active predators capable of feeding on swiftly moving prey,[2]with preserved stomach contents of a specimen ofHybodus hauffianusindicating that they fed onbelemnites.[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

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Fossil ofHamiltonichthysa primitive hybodont from the Carboniferous of North America

The earliest hybodont remains are from the latestDevonian(Famennian) of Iran, belonging to the genusRoongodus,[31]as well as remains assigned toLissodusof the same age from Belgium.[32]Although the first fossils of hybodonts are from the latest Devonian, they likely branched off from neoselachians (modern sharks) during the early Devonian.[33]Carboniferous hybodonts include both durophagous and non-durophagous forms, while durophagous forms were dominant during the Permian period.[32]By the Permian period, hybodonts had a global distribution.[32][34][35]ThePermian-Triassic extinction eventonly had a limited effect on hybodont diversity.[36]Maximum hybodont diversity is observed during the Triassic. During the Triassic and Early Jurassic, hybodontiforms were the dominant elasmobranchs in both marine and non-marine environments.[20]A shift in hybodonts was seen during the Middle Jurassic, a transition between the distinctly different assemblages seen in the Triassic – Early Jurassic and the Late Jurassic – Cretaceous.[20]As neoselachians (group of modern sharks) diversified further during the Late Jurassic, hybodontiforms became less prevalent in open marine conditions but remained diverse in fluvial and restricted settings during the Cretaceous.[20]By the end of the Cretaceous, hybodonts had declined to only a handful of species,[37]including members ofLonchidion[38],andMeristodonoides.[39]The last hybodonts disappeared, seemingly abruptly, as part of theCretaceous-Paleogene extinction event.[37]

Families and genera

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

References

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  1. ^Zittel, K. von, 1911, Grunzuege der Palaontologie, 2 ed. II. Abt. Vertebrata, vii + 598 pp. R. Oldenburg Verlag, Muchen, Berlin.
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