Hybodontoidea
| Hybodontiformes Temporal range: Carboniferous–Miocene |
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| Ptychodus mortoni, a giant 7 m long durophagous Late Cretaceous shark | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Superorder: | Selachimorpha |
| Order: |
†Hybodontiformes Owen, 1846 |
| Families | |
Hybodontiformes, also called hybodonts, are an extinct subset of Elasmobranchii (sharks, skates and rays) which existed from the Devonian to the Miocene. They form the group of sharks closest to neoselachians, the clade of modern sharks and rays. Hybodonts were named and are distinguished based on their conical tooth shape. They comprised the main group of Jurassic sharks in Europe and North America. They survived into the Late Cretaceous before going extinct, possibly due to competition from other sharks, though forms like Miosynechodus endured as recently as the Miocene.Lonchidion was one of the last hybodonts— its distinctive serrated fine spines occur in freshwater deposits from Wyoming alongside the fossils of the last dinosaurs, including Tyrannosaurus rex and Triceratops.Hybodontiformes are identified in the fossil record predominantly based on distinct teeth and fin spines. They were known to live in both fresh and salt water environments.
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.
Hybodontiformes are a type of elasmobranch shark, just outside Neoselachians. Hybodontiformes are classified in the Euselachii along with Xenacanthiformes, Ctenacanthiformes and Neoselachii. The order Hybodontiformes includes the families Polyarcodontidae, Hybodontidae and Ptychodontidae. Lissodus sp. is a common example of Polyacrodontidae, Hybodus is an example of Hybodontidae, and Ptychodus mortoni is an example of Ptychodontidae.
Hybodonts were first described in the nineteenth century based on isolated fossil teeth (Agassiz, 1837). The earliest hybodont remains are from the Carboniferous and include Tristychius and other fishes from the Calciferous Sandstone of Scotland. Hybodonts were first separated from living sharks by Zittel (1911). Although the first fossils of hybodonts are from the Carboniferous, they likely branched off from neoselachians (modern sharks) during the early Devonian. The group now called Hybodontiformes includes many species, with examples such as Hybodus, Acrodus, Asteracanthus, Lonchidion, and Lissodus. Hybodont samples have been recovered from Permian deposits from Oman, indicating that hybodonts lived in the Neotethys Ocean during the Permian Period. This study combined with others from Texas suggest that hybodonts were well established, and in some places dominant, during the Permian. In general, the Permian record of hybodonts is limited. It was initially hypothesized that hybodont diversity was not significantly impacted by the end-Permian extinction, instead it was thought that diversity of Permian hybodonts declined over the 50 million years before the end-Permian extinction. However, recent samples found in Oman suggests that Permian hybodont diversity extended until the end-Permian, suggesting the extinction was more impactful than previously thought. Fossils from the Lower Triassic Vega-Phroso Siltstone Member of the Sulphur Mountain Formation of Alberta, Canada show well preserved specimens of Wapitiodus aplopagus which survived the extinction and was abundant in the Early Triassic. Maximum hybodont diversity is observed during the Triassic. During the Triassic and Early Jurassic, hybodontiforms were the dominant selachians in both marine and non-marine environments. A study of Middle Jurassic fossils from England analyzed 20 species from 11 genera suggesting that hybodonts flourished at that time. 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. 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. By the Cretaceous, hybodontiforms were primarily (though not solely) restricted to freshwater settings. They remained successful during the Cretaceous by adapting to freshwater conditions, for example seven genera were found in freshwater deposits from Thailand. The end-Cretaceous extinction of hybodont sharks may have been caused more by competition with other sharks than by the meteorite impact and volcanic eruptions cited to be the main cause of this extinction event. Most other sharks were not significantly affected by the end-Cretaceous extinction, also suggesting that competition lead to the demise of hybodonts.
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