Among the macroinvertebrate feeders are those that practice durophagy
adapted to feed on shelled animals. Modern forms include
bullhead sharks (Heterodontus spp.) andmyliobatid rays (Summers,
2000; Compagno, 2001), and some extinct forms include the enigmatic
ptychodontid sharks (Elasmobranchii: Hybodontiformes?)
that lived during the Cretaceous and are characterized by pavement-
like dental plates consisting of juxtaposed rows of teeth
(Shimada et al., 2009).