Bullhead shark

Bullhead shark Temporal range: 183–0 Ma Early Toarcian to Present. PreЄ Є O S D C P T J K Pg N
Horn shark, Heterodontus francisci
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Superorder:	Selachimorpha
Order:	Heterodontiformes L. S. Berg, 1940
Family:	Heterodontidae J. E. Gray, 1851
Genus:	Heterodontus Blainville, 1816
Species
See text.

The Bullhead sharks are a small order (Heterodontiformes) of basal modern sharks (Neoselachii). There are nine living species in a single genus, Heterodontus, in the family Heterodontidae. All are relatively small, with the largest species being just 150 centimetres (59 in) in adult length. They are bottom feeders in tropical and subtropical waters.

The Heterodontiforms appear in the fossil record in the Early Jurassic, well before any of the other Galeomorphii, a group that includes all modern sharks except the dogfish and its relatives. However, they have never been common, and it is likely that their origin lies even further back.

Description

Bullhead shark egg case

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 folds are present; but the rostral process of the neurocranium (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 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.

Bullhead Sharks have distinctive small spikes on the front of their dorsal fins. These are rumoured to be poisonous, but no further scientific tests have been done to prove this rumor true or false.

Species

There are nine living species of bullhead shark, with another potential undescribed species in Baja California:

• 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, 1831) (Zebra bullhead shark)
• Heterodontus sp. X J. I. Castro (Cryptic hornshark)

• Heterodontus philippi.jpg

  A Port Jackson shark, Heterodontus portusjacksoni

• Heterodontus portusjacksoni 1.jpg

  Dentition and oronasal grooves of a Port Jackson shark

• Heterodontus zebra.jpg

  Zebra bullhead shark, Heterodontus zebra

See also

• List of prehistoric cartilaginous fish

Further reading

• Compagno, Leonard (2002) Sharks of the World: Bullhead, mackerel and carpet sharks Volume 2, FAO Species Catalogue, Rome. ISBN 92-5-104543-7.
• Froese, Rainer, and Daniel Pauly, eds. (2011). Species of Heterodontus in FishBase. February 2011 version.