Order HYBODONTIFORMES. Hybodontids have the features given above for the ctenacanthiforms. They differ, among other features, in their internal fin structure. Males have hooked cephalic spines above the eye that may have functioned as claspers during copulation. Hybodonts might have been as diverse in feeding and related behavioral strategies as is found among living sharks and rays (Maisey and de Carvalho, 1997). Some were several meters long, others only about 15 cm.
Hybodonts are probably the closest extinct sister group to the neoselachi-ans (Maisey et al., 2004), and this has been expressed here by giving the two groups equal rank. As noted by Maisey et al. (2004), of all elasmobranchs, only these two lineages, the hybodonts and neoselachians, are known to have survived well into the Mesozoic (the other lineages becoming extinct in the Paleozoic, many in the Permo-Triassic mass extinction and the others during the Triassic). Only the neoselachians survived into the Cenozoic. Cappetta (1987) and Cappetta et al. (1993) recognized several families for the taxa given below.
One (Hybodontidae) or more families with the following generic examples from Maisey (1982, 1989, 1991), Cappetta (1987), and Cappetta et al. (1993): Acrodus, Asteracanthus, Hamiltonichthys, Hybodus, Lissodus, Lonchidion (see Arratia et al., 2002), Palaeobates, Polyacrodus, Protacrodus, Pseudodalatias, Ptychodus, Steinbachodus, and Tribodus. Mississippian to Cretaceous (the dominant selachians of the Triassic and Jurassic).
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