Hybodontiformes

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 (Agassiz, 1837). Hybodonts were first separated from living sharks by Zittel (1911). Although historically argued to have a close relationship with the modern shark order Heterodontiformes, this has been refuted. Hybodonts are divided into a number of families, but the higher level taxonomy of hybodonts, especially Mesozoic taxa, is poorly resolved. Simplified cladogram of chondrichthyan relationships following several studies. }}}}}}}} == Description ==

''|left The largest hybodonts reached lengths of , Hybodonts had a generally robust bodyform. Due to their 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. showing sexual dimorphism with fin claspers and cephalic spines present in males (below) but absent in females (above)Hybodonts are recognized as having teeth with a prominent cusp which is higher than lateral cusplets. Hybodont teeth are often preserved as incomplete fossils because the base of the tooth is not well attached to the crown. 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. 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'. 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. Similar fin spines are also found in many extinct chondrichthyan groups as well as in some modern sharks like Heterodontus and squalids. Male hybodonts had either one or two pairs of cephalic spines on their heads, a characteristic distinctive to hybodonts. These spines, while of variable placement, 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. Like living sharks and rays, the skin of hybodonts was covered with dermal denticles. 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. {{gallery|Durnonovariaodus.png|Teeth of Durnovariaodus, a member of the family Hybodontidae|Strophodus tooth.png|Jaw fragment of Strophodus a specialised durophagous hybodont|Asteracanthus jaws.jpg|Jaws of Asteracanthus, showing the arrangement of the teeth in jaws. The teeth were designed for grasping|Acrodus dentition Meride cast (cropped).jpg|width=300|height=240|Preserved dentition of the acrodontid Acrodus, which had low, rounded teeth used in durophagy|File:Planohybodus.jpg == Ecology ==

) attached to a Neocalamites'' stem in an estuarine environment. Art by Michael Rothman Hybodont fossils are found in depositional environments ranging from marine to fluvial (river deposits). Hybodonts inhabited freshwater environments from early in their evolutionary history, spanning from the Carboniferous onwards. Based on isotopic analysis, some species of hybodonts are likely to have permanently lived in freshwater environments, while others may have migrated between marine and freshwater environments. 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. It has been suggested that male hybodonts used their cephalic spines to grip females during mating. 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. 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. with preserved stomach contents of a specimen of Hybodus hauffianus indicating that they fed on belemnites (a type of extinct squid-like cephalopod). Hybodonts have a wide variety of tooth shapes. This variety suggests that they took advantage of multiple food sources. == Evolutionary history ==

'' a primitive hybodont from the Carboniferous of North America The earliest hybodont remains are from the latest Devonian (Famennian, ~ 360 million years ago) of Iran, belonging to the genus Roongodus, as well as remains assigned to Lissodus of the same age from Belgium. Carboniferous hybodonts include both durophagous and non-durophagous forms, while durophagous forms were dominant during the Permian period. The Permian-Triassic extinction event only had a limited effect on hybodont diversity. 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. 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. By the end of the Cretaceous, hybodonts had declined to only a handful of species, including members of Lonchidion, and Meristodonoides. The last hybodonts disappeared, seemingly abruptly, as part of the Cretaceous-Paleogene extinction event approximately 66 million years ago. == Families and genera ==

The taxonomy of hybodonts is considered poorly resolved, • Baharyodon • Diplolonchidion • Vectiselachos • Hylaeobatis • Isanodus • Parvodus • Lissodus? • Lonchidion • Lonchidionoides • Luopingselache • Jiaodontus • Pristrisodus • Distobatidae • Distobatus • Reticulodus • Tribodus? • Aegyptobatus • Acrodontidae • Acrodus • Strophodus? • Hybodontidae • Dicrenodus • Egertonodus • Hybodus • Meristodonoides • Planohybodus • Priohybodus • Sphenonchus • Durnonovariaodus • Crassodus • Incertae sedis • Tribodus • Strophodus • Asteracanthus • Roongodus • Polyacrodus • Palaeobates • Bdellodus • Thaiodus • Acrorhizodus • Khoratodus • Arctacanthus • Reesodus • Steinbachodus • Onychoselache • Omanoselache • Pororhiza • Mukdahanodus • Secarodus • Hamiltonichthys • Gansuselache • Dabasacanthus • Teresodus • Diablodontus • Gunnellodus? • Heteroptychodus • Lissodus • Columnaodus • Carinacanthus • Form genera • Palaeoxyris (genus used for the egg capsules of hybodonts) ==References==