The bullhead sharks are members of the genus Heterodontus, the only members of the family Heterodontidae and only living members of the order Heterodontiformes. All are relatively small, with the largest species reaching just 1.65m (05.41feet) in maximum length. They are bottom feeders in tropical and subtropical waters.
The Heterodontiforms appear in the fossil record in the Early Jurassic.[1] The oldest fossils of the modern genus date to the Late Jurassic. Despite the very ancient origins of this genus and its abundance in the fossil record, phylogenetic evidence indicates that all extant species in the genus arose from a single common ancestor that survived the Cretaceous-Paleogene extinction, with diversification into modern species only starting around the mid-Eocene.[2]
The bullhead sharks are characterised by a broad head, heavy brow, stubby snout and small mouth. The mouth is located entirely anterior to the eye sockets, due to their protruding jaw structure. They have a distinct groove from their nostrils to their mouths. They grow to about a meter in size, with the largest species being that of the Port Jackson shark. The eyes lack a nictitating membrane. A spiracle is present, but small. The dorsal ends of the fourth and fifth branchial arches are attached, but not fused into a "pickaxe" as in lamniform sharks. Heterodontiforms have two dorsal fins, with fin spines, as well as an anal fin. The dorsal and anal fins also contain basal cartilages, not just fin rays.[1]
Ten living species of bullhead shark have been described: