Bullhead shark

Bullhead sharks Temporal range: 174–0 Ma PreЄ Є Є O O S S D D C C P P T T J J K K Pg Pg N N Late Toarcian to Present
File:Heterodontus portusjacksoni francisci 060417w.jpg
Two Port Jackson sharks (Heterodontus portusjacksoni)
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 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.

Description

File:Oeufs002b,55.png

Bullhead shark egg

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.

The eyes lack a nictitating membrane. 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.

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 & 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 (Jose I. Castro) (Cryptic Horn Shark)

See also

• List of prehistoric cartilaginous fish

References

• "Heterodontus". Integrated Taxonomic Information System. Retrieved 4 May 2006. 
• Froese, Rainer, and Daniel Pauly, eds. (2011). Species of Heterodontus in FishBase. February 2011 version.