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 led to the demise of hybodonts. Habitat Hybodont teeth fossils are found in depositional environments ranging from marine to fluvial (river deposits). When they first evolved they inhabited both marine and freshwater systems. While hybodonts lived in freshwater throughout their existence, an example of hybodonts moving into more restricted conditions comes from Middle Jurassic samples found in lagoonal and other enclosed depositional settings. 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. Behavior Hybodonts were likely slow swimmers and used their paired fins for steering and stabilization. Hybodus, a typical hybodontiform, was thought to be a slow swimmer but capable of occasional bursts on speed, making it an active predator of fast moving prey. Hybodonts have a wide variety of tooth shapes. This variety suggests that they took advantage of multiple food sources. It is thought that some hybodonts which had wider, flatter, teeth specialized in crushing hard-shelled prey (durophagy), evidence for this includes well-developed wear facets on teeth from Lissodus. Species described from Thailand have a range of teeth shapes, suggesting multiple feeding habits. Bulbous teeth were used for crushing hard shelled bottom-dwelling prey. Others were opportunistic feeders and were species that had a diet of large soft-bodied prey. Little is known about the reproductive habits of hybodonts. One study found abundant fossil teeth and eggs sacks in freshwater lake deposits from the Triassic of Kyrgyzstan. The site was interpreted as an ancient shark nursery based on the abundance of eggs and juvenile fossils and the limited number of adult specimens. References External links https://web.archive.org/web/20081106004709/http://palaeos.com/Vertebrates/Units/070Chondrichthyes/070.600.html#Hybodontiformes http://www.helsinki.fi/~mhaaramo/metazoa/deuterostoma/chordata/chondrichthyes/elasmobranchii/hybodontiformes/hybodontiformes.html Prehistoric cartilaginous fish orders Carboniferous first appearances Pennsylvanian taxonomic orders Cisuralian taxonomic orders Guadalupian taxonomic orders Lopingian taxonomic orders Early Triassic taxonomic