''Hybodus'' (from , 'crooked' and 'tooth') is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

hybodont Hybodontiformes, commonly called hybodonts, are an extinct group of shark-like cartilaginous fish (chondrichthyans) which existed from the late Devonian to the Late Cretaceous. Hybodonts share a close common ancestry with modern sharks and rays ( ...

. Species closely related to the type species ''Hybodus reticulatus'' lived during the

Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic� ...

epoch. Numerous species have been assigned to ''Hybodus'' spanning a large period of time, and it is currently considered a

wastebasket taxon Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists to refer to a taxon that has the purpose of classifying organisms that do not fit anywhere else. They are typically defined by e ...

that is 'broadly

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

' and requires reexamination.

Description

''Hybodus'' species typically grew to about in length, with larger specimens of ''H. hauffianus'' reaching about . It possessed a streamlined body shape similar to modern sharks, with two similarly sized

dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...

s. As in other

Hybodontiformes Hybodontiformes, commonly called hybodonts, are an extinct group of shark-like cartilaginous fish (chondrichthyans) which existed from the late Devonian to the Late Cretaceous. Hybodonts share a close common ancestry with modern sharks and rays ( ...

dentin Dentin ( ) (American English) or dentine ( or ) (British English) () is a calcified tissue (biology), tissue of the body and, along with tooth enamel, enamel, cementum, and pulp (tooth), pulp, is one of the four major components of teeth. It i ...

ous fin spines were present on the dorsal fins of ''Hybodus,'' which in this genus exhibit a rib-like ornamentation located towards the tip of the spine, with rows of hooked denticles present on the posterior side. The males possessed

clasper In biology, a clasper is a male anatomical structure found in some groups of animals, used in mating. Male cartilaginous fish have claspers formed from the posterior portion of their pelvic fin which serve to channel semen into the female's ...

s, specialized organs that directly insert sperm into the female, and which are still present in modern sharks.

Species

Several ''Hybodus'' species, including ''H. butleri'', ''H. rajkovichi'', and ''H. montanensis'', were later reassigned to '' Meristodonoides''. ''H. basanus'' and ''H. fraasi'' are now included in the genus '' Egertonodus,'' though the placement of the latter in the ''Egertonodus'' is considered tentative, due to the strong differences in tooth morphology between the two species. ''H. obtusus'' represents a junior synonym of '' Asteracanthus ornatissimus''. Multiple Triassic species previously assigned to ''Hybodus'' including ''H. microdus'', ''H. yohi'', ''H. zuodengensis'' and ''H. bugarensis'' are not hybodonts and most likely belong to an entirely separate order

Synechodontiformes Synechodontiformes is an extinct order of prehistoric shark-like cartilaginous fish, known from the Permian to the Paleogene. They are considered to be members of Neoselachii, the group that contains modern sharks and rays. Their placement ...

. Two new species from China, ''H. xinzhuangensis'' and ''H. chuanjieensis'' are named in 2018, and ''H. houtienensis'' is considered, while other species from China and Thailand are no longer part of the genus and requires reassessment. However, the only two species that should be retained within the genus ''Hybodus'' are the type species ''H. reticulatus'' and the other species ''H. hauffianus''. The problem is that even those two species require reassessment.

Paleobiology

''Hybodus'' is thought to have been an active predator which was capable of consuming swiftly moving prey, probably predominantly

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

s and, to a lesser extent, fish. Based on fossilized stomach contents, it has been proposed that ''Hybodus'' was a more active hunter than its close relatives like ''Asteracanthus,'' which likely fed on

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". ...

prey. A preserved specimen of ''Hybodus hauffianus'' has been found with over 100

belemnite Belemnitida (or belemnites) is an extinct order (biology), order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous (And possibly the Eocene). Unlike squid, belemnites had an internal skeleton that made up the cone ...

(a type of extinct squid-like cephalopod) rostra (hard mineralized internal elements) in its stomach, which may have resulted in its death. ''Hybodus varied dentition would have allowed it to opportunistically exploit a variety of food sources; sharper teeth would have been used to catch slippery prey, while flatter teeth probably helped them crush shelled animals. The large spines on the first and second dorsal fins were likely defensive structures intended to protect against attacks from ''Hybodus''' own predators.

References

Sources

* * {{Taxonbar, from=Q132688 Hybodontiformes Prehistoric cartilaginous fish genera Prehistoric fish of Africa Fossils of Uzbekistan Bissekty Formation Prehistoric fish of Europe Hasle Formation Prehistoric fish of South America Taxa named by Louis Agassiz Fossil taxa described in 1837 Jurassic cartilaginous fish