The bullhead sharks are a small order (Heterodontiformes) of very basal (primitive) modern sharks (Neoselachii). There are eight living species in a single genus, Heterodontus, in the family Heterodontidae. All are relatively small (50 to 150 cm) bottom feeders in tropical and subtropical waters. Scientific classification or biological classification refers to how biologists group and categorize extinct and living species of organisms. ... Orders Hexanchiformes Squaliformes Pristiophoriformes Squatiniformes Heterodontiformes Orectolobiformes Carcharhiniformes Lamniformes Sharks are a group (superorder Selachimorpha) of fish, with a full cartilaginous skeleton, a streamlined body plan with between 5 and 7 gill slits along the sides (most often) or side of the head (the first modified slit is behind the...

The symbol '$' below indicates a synapomorphy, (a character believed to be uniquely present in the last common ancestor of all heterodontiforms).

The heterodontiforms are morphologically rather distinctive. The mouth is located entirely anterior to the orbits. Labial cartilages are found in the most anterior part of the mouth. Nasoral grooves are present, connecting the external nares to the mouth. The nasal capsules are "trumpet-shaped" & well-separated from orbits. $ Circumnarial skin folds present; but $ the rostral process of neurocranium (braincase) absent, although a precerebral fossa is present. Finally, the braincase bears a supraorbital crest. In zootomy, several terms are used to describe the location of organs and other structures in the body of bilateral animals. ...

The eyes lack a nictitating fold. A spiracle is present, but small. The dorsal ends of branchial arches 4 and 5 are attached, but not fused into a "pickaxe" as in lamniform sharks. Heterodontiforms have 2 dorsal fins, with fin spines, as well as an anal fin. The dorsal and anal fin also contain basal cartilages, not just fin rays. Spiracles are small openings on the surface of animals that usually lead to respiratory systems. ... The word dorsal can refer to many different things. ... Families Odontaspididae (sand tigers) Mitsukurinidae (goblin shark) Pseudocarchariidae (crocodile shark) Megachasmidae (megamouth shark) Alopiidae (thresher sharks) Cetorhinidae (basking shark) Lamnidae (mackerel sharks) Great Lamniformes is an order of sharks commonly known as the mackerel sharks. ... Dorsal Fin of the Orca A dorsal fin is a fin located on the backs of fishes, whales, dolphins and porpoises. ... Fish anatomy is primarily governed by the physical characteristics of water, which is much denser than air, holds a relatively small amount of dissolved oxygen, and absorbs light more than does air. ... Cartilage is a type of dense connective tissue. ...

The Heterodontiforms appear in the fossil record in the Early Jurassic, well before any of the other Galeomorphii, a group which includes all modern sharks except the dogfish and its relatives. However, they have never been common, and it is likely their origin lies even further back. The Jurassic period is a major unit of the geologic timescale that extends from about 200 million years BP at the end of the Triassic to 146 million years BP at the beginning of the Cretaceous. ... Spiny Dogfish, one of the many Dogfish species The name dogfish is applied to a number of types of small sharks found in the northeast Atlantic, Pacific, and Mediterranean, especially three principal families – Scyliorhinidae, Squalidae and Squalidae. ...

Classification

The genus Heterodontus contains eight species:

• Heterodontus francisci (Girard, 1855) (Horn shark) [1]
• Heterodontus galeatus (Günther, 1870) (Crested bullhead shark) [2]
• Heterodontus japonicus Maclay & Macleay, 1884 (Japanese bullhead shark) [3]
• Heterodontus mexicanus Taylor & Castro-Aguirre, 1972 (Mexican hornshark) [4]
• Heterodontus portusjacksonj (Meyer, 1793) (Port Jackson shark) [5]
• Heterodontus quoyi (Fréminville, 1840) (Galapagos bullhead shark) [6]
• Heterodontus ramalheira (Smith, 1949) (Whitespotted bullhead shark) [7]
• Heterodontus zebra (Gray, 1831) (Zebra bullhead shark) [8]