Bullhead shark
The bullhead sharks are a small order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 (Heterodontiformes) of basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

 modern shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s (Neoselachii
Neoselachii are a subclass of class Chondrichthyes. It contains two kinds of fishes: sharks and rays. The other subclass is Holocephali....

). 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 (59.1 in) in adult 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 system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

, well before any of the other Galeomorphii, a group which includes all modern sharks except the dogfish
Squalidae is the family of dogfish sharks. They are found in the Atlantic, Pacific, and Indian oceans, from tropical equatorial climates to the Arctic and Antarctic....

 and its relatives. However, they have never been common, and it is likely their origin lies even further back.


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 fold
Skin fold
Skin folds are areas of skin where it folds. 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 individual development of an organism or by arbitrary application of force to...

s 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
Nictitating membrane
The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have a full nictitating membrane; in many mammals, there is a small...

. A spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...

 is present, but small. The dorsal
Dorsum (biology)
In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...

 ends of branchial arches 4 and 5 are attached, but not fused into a "pickaxe" as in lamniform sharks
Lamniformes is an order of sharks commonly known as mackerel sharks . It includes some of the most familiar species of sharks, such as the great white shark, as well as more unusual representatives, such as the goblin shark and the megamouth shark.Members of the order are distinguished by...

. Heterodontiforms have 2 dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

s, with fin spines, as well as an anal fin. The dorsal and anal fin also contain basal cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

s, 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.


There are nine living species of bullhead shark, with another potential undescribed species in Baja California:
  • Heterodontus francisci (Girard
    Charles Frédéric Girard
    Charles Frédéric Girard was a French biologist specializing in ichthyology and herpetology.Born in Mulhouse, France, he studied at the College of Neuchâtel, Switzerland as a student of Louis Agassiz. In 1847, he accompanied Agassiz as his assistant to Harvard...

    , 1855)
    (Horn shark)
  • Heterodontus galeatus (Günther, 1870) (Crested bullhead shark)
  • Heterodontus japonicus Maclay & Macleay
    William John Macleay
    Sir William John Macleay . was an Australian politician, zoologist and naturalist.-Early life:Macleay was born at Wick, Caithness, Scotland, second son of Kenneth Macleay of Keiss and his wife Barbara, née Horne...

    , 1884
    (Japanese bullhead shark)
  • Heterodontus mexicanus Taylor & Castro-Aguirre, 1972 (Mexican hornshark)
  • Heterodontus omanensis Z. H. Baldwin, 2005 (Oman bullhead shark)
  • Heterodontus portusjacksoni (F. A. A. Meyer
    Friedrich Albrecht Anton Meyer
    Friedrich Albrecht Anton Meyer was a German doctor and naturalist.His academic thesis in Göttingen was Dissertatio inauguralis medico-therapeutica De cortice angusturae....

    , 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
    John Edward Gray, FRS was a British zoologist. He was the elder brother of George Robert Gray and son of the pharmacologist and botanist Samuel Frederick Gray ....

    , 1831)
    (Zebra bullhead shark)
  • Heterodontus sp. X (Jose I. Castro) (Cryptic Horn Shark)
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