The bullhead sharks are a small order (Heterodontiformes ) of modern

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s (

Neoselachii Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including sharks (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of gi ...

). The nine living species are placed in a single genus, ''Heterodontus'', in the family Heterodontidae. All are relatively small, with the largest species reaching just in maximum length. They are bottom feeders in tropical and subtropical waters. The Heterodontiforms appear in the fossil record in the Early

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

, well before any of the other

Galeomorphii Galeomorphii is a superorder of cartilaginous fishes which includes all modern sharks except the dogfish and its relatives. They are sometimes called galea or galean sharks. There are about 300 living species in 23 families. Galean sharks ar ...

, a group that includes all modern sharks except the dogfish and its relatives. However, they have never been common, and their origin probably lies even further back.

Description

The bullhead sharks 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 and well-separated from orbits. Circumnarial

skin fold Skin folds or skinfolds are areas of skin that are naturally folded. Many skin folds are distinct, heritable anatomical features, and may be used for identification of animal species, while others are non-specific and may be produced either by ind ...

s are present, but the rostral process of the

neurocranium In human anatomy, the neurocranium, also known as the braincase, brainpan, or brain-pan is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calvaria or skul ...

(braincase) is absent, although a precerebral fossa is present. Finally, the braincase bears a supraorbital crest. The eyes lack a nictitating membrane. A spiracle is present, but small. The

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal c ...

ends of the fourth and fifth branchial arches are attached, but not fused into a "pickaxe" as in lamniform sharks. Heterodontiforms have two

dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through c ...

s, with fin spines, as well as an anal fin. The dorsal and anal fins also contain basal cartilages, not just fin rays.

Species

Nine living species of bullhead shark have been described: * '' Heterodontus francisci'' ( Girard, 1855) (horn shark) * '' Heterodontus galeatus'' ( Günther, 1870) (crested bullhead shark) * '' Heterodontus japonicus'' ( Maclay & W. J. Macleay, 1884) (Japanese bullhead shark) * '' Heterodontus mexicanus'' ( L. R. Taylor & Castro-Aguirre, 1972) (Mexican hornshark) * '' Heterodontus omanensis'' ( Z. H. Baldwin, 2005) (Oman bullhead shark) * '' Heterodontus portusjacksoni'' ( F. A. A. Meyer, 1793) (Port Jackson shark) * '' Heterodontus quoyi'' ( Fréminville, 1840) (Galapagos bullhead shark) * '' Heterodontus ramalheira'' ( J. L. B. Smith, 1949) (whitespotted bullhead shark) * '' Heterodontus zebra'' (

J. E. Gray John Edward Gray, Fellow of the Royal Society, FRS (12 February 1800 – 7 March 1875) was a British zoology, zoologist. He was the elder brother of zoologist George Robert Gray and son of the pharmacologist and botanist Samuel Frederick Gray ...

, 1831) (zebra bullhead shark) File:Heterodontus philippi.jpg, A

Port Jackson shark The Port Jackson shark (''Heterodontus portusjacksoni'') is a nocturnal, oviparous (egg laying) type of bullhead shark of the family Heterodontidae, found in the coastal region of southern Australia, including the waters off Port Jackson. It has ...

, ''Heterodontus portusjacksoni'' File:Heterodontus portusjacksoni 1.jpg, Dentition and oronasal grooves of a Port Jackson shark File:Heterodontus zebra.jpg,

Zebra bullhead shark The zebra bullhead shark (''Heterodontus zebra'') is a bullhead shark of the family (biology), family Heterodontidae found in the central Indo-Pacific between latitudes 40th parallel north, 40°N and 20th parallel south, 20°S, from Japan and Kor ...

, ''Heterodontus zebra''

Description

Species

See also

References

Further reading