Temporal range: 183–0 Ma Early Toarcian to Present. 
|Horn shark, Heterodontus francisci|
L. S. Berg, 1940
J. E. Gray, 1851
The bullhead sharks are a small order (Heterodontiformes) of basal modern sharks (Neoselachii). The nine living species are placed in a single genus, Heterodontus, in the family Heterodontidae. All are relatively small, with the largest species being just 150 cm (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 their origin probably lies even further back.
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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.
Nine living species of bullhead shark are described, 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)
- Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (Chondrichthyes entry)". Bulletins of American Paleontology (ocean) 364: 560. Retrieved 2008-01-09.
- . N.p.. Web. 10 Jun 2013. <http://paleodb.org/cgi-bin/bridge.pl?a=checkTaxonInfo&taxon_no=252518&is_real_user=1>.
- Compagno, Leonard (2002) Sharks of the World: Bullhead, mackerel and carpet sharks Volume 2, FAO Species Catalogue, Rome. ISBN 92-5-104543-7.
- "Heterodontus". Integrated Taxonomic Information System. Retrieved 4 May 2006.
- Froese, Rainer, and Daniel Pauly, eds. (2011). Species of Heterodontus in FishBase. February 2011 version.