Temporal range: Early Jurassic, 199–196 Ma
|Cast of SAM-PK-K1332, University of California Museum of Paleontology|
Crompton & Charig, 1962
Heterodontosaurus (meaning "different toothed lizard") is a genus of small herbivorous dinosaur with prominent canine teeth which lived in the Early Jurassic of South Africa. It was similar to a hypsilophodont in shape, and ate plants, despite its canines.
Heterodontosaurus was a small, fleetfooted ornithischian classified inside the family Heterodontosauridae. The family contains some of the smallest known ornithischians – the North American Fruitadens, for example, reached a length of only 65 to 75 cm. Heterodontosaurus was amongst the largest known members of its family, reaching a length of between 1 to 1.75 m and weighing 1.8 or 10 kg. Only the South African Lycorhinus might have been larger.
The skull was robustly built and triangular in side view. The front of the jaws where covered by a toothless horny beak. The upper beak was carried by the premaxilla and the lower beak by the predentary, which in ornithischians are the foremost bones of the upper and lower jaw, respectively. The eye openings were proportionally large and almost circular in shape, while the external nostril openings were small. A large spur-like bone, the palpebral, protruded backwards into the eye opening. Below the eye socket, the jugal bone gave rise to a sidewards projecting horn like process. The antorbital fossa, a large depression between the eye and nostril openings, contained two smaller openings. Ventrally, the antorbital fossa was bounded by a prominent bony ridge, to which the animal's fleshy cheek would have been attached. Behind the eye opening, there was the proportionally large lower temporal fenestra, which in Heterodontosaurus was egg-shaped and tilted back. The elliptical upper temporal fenestra was visible only in top view of the skull. The left and right upper temporal fenestrae were separated by a pronounced sagittal crest, which in the living animal would have provided attachment sides for the jaw musculature.
The neck consisted of nine cervical vertebrae, which would have followed an S-curve, as indicated by the shape of the vertebral bodies in side view: while the vertebral bodies of the anterior cervical vertebrae are shaped like a parallelogram, those of the middle were rectangular and those of the posterior showed a trapezoid shape. The trunk was short, consisting of 12 dorsal and 6 fused sacral vertebrae. The tail was long compared to the body; although incompletely known, it would probably have consisted of 34 to 37 caudal vertebrae. The dorsal spine was stiffened by ossified tendons, beginning with the forth dorsal vertebrae. This feature is present in many other ornithischians and probably countered stress caused by bending forces acting on the spine during bipedal locomotion. Different to many other ornithischians, the tail of Heterodontosaurus lacked ossified tendons, and therefore probably was flexible. It had a long, narrow pelvis and a pubis which resembled those possessed by more advanced ornithischians.
The forelimbs were robustly built and proportionally long, measuring 70% of the length of the hind limbs. The radius of the forearm measured 70% of the length of the humerus. The hand was large, approaching the humerus in length, and showed five fingers equipped for grasping. The bone in the foot and ankle were fused in a manner reminiscent of those in birds. The second finger was the longest, followed by the third and the first finger (the thumb). The first three fingers ended in large and strong claws. The forth and fifth finger were strongly reduced and possibly functionless. The phalangeal formula, which states the number of finger bones in each finger starting from the first, was 2-3-4-3-2.
The hindlimbs were long and gracile and ended in four toes, of which only the second, third and forth made contact with the ground. Uniquely for ornithischians, several bones of the leg and foot were fused: The tibia and fibula were fused with upper tarsal bones (astragalus and calcaneus), forming a tibiotarsus, while the lower tarsal bones were fused with the metatarsal bones, forming a tarsometatarsus. This constellation can be also found in today's birds, where it has evolved independently. The tibiotarsus was ca. 30% longer than the femur.
Following the discovery of the related Tianyulong in 2009, which was preserved with long, filamentous integuments, Heterodontosaurus has also been depicted with such structures, for example in publications by Gregory S. Paul and Paul Sereno. Sereno also published a restoration of Heterodontosaurus with a hypothetical display structure located above the nasal fossa, though the function of the fossa is unknown. A similar fossa is also seen in Tianyulong, Agilisaurus, and Eoraptor.
Another feature is the specialization of teeth which gave rise to the animal's name. Most dinosaurs (and indeed most reptiles) have a single type of tooth in their jaws, while Heterodontosaurus had three. At the front of the jaw beside the beak were small teeth likely used for chopping off leaves and stems.
Next in the jaw was a large pair of tusks whose purpose is unknown, but it is speculated that they were used as sexual displays (where the tusks could have been used as weapons by rival males in disputes over mates and territories) or to break open prehistoric termite mounds. The final type of teeth were tall and squared off. This type of teeth was well adapted for chewing. Fleshy cheeks helped keep the food in the mouth while chewing occurred. Chewing is relatively common in dinosaurs, but uncommon for other groups of reptiles.
The holotype specimen of Heterodontosaurus tucki (SAM-PK-K1337, housed in the South African Museum) was discovered during the 1961-1962 British-South African expedition to South Africa and Basutoland (former name of Lesotho). It was excavated on a mountain at an altitude of about 1890 m (6,200 ft), at a locality called Tyinindini, in the district of Transkei (or Herschel) in the Cape Province of South Africa. The animal was scientifically described and named in 1962 by palaeontologists Alfred W. Crompton from South Africa and Alan J. Charig from the United Kingdom. The genus name refers to the differentiated dentition (unusual for an ornithischian dinosaur), and the specific name honours G. C. Tuck, a director of Austin Motor Company, who assisted the expedition. The specimen was not fully prepared by the time of publication, so only the front part of the skull and lower jaw was described, and the authors conceded that their description was preliminary, serving mainly to name the animal. It was considered an important discovery, as few early ornithiscians were known at the time. Preparation was very time consuming, since the bones were covered in a thin, very hard, ferruginous layer (containing haematite), only removable with a diamond saw (which damaged the specimen). The holotype specimen consists of a crushed but nearly complete skull; associated postcranial remains mentioned in the original description could not be located in 2011.
In 1966, a second specimen of Heterodontosaurus (SAM-PK-K1332) was discovered at the Voyizane locality, in the Elliot Formation (Upper Red Beds) of the Stormberg Group (a series of formations), 1,770 m above sea level, on Krommespruit Mountain. This specimen included both the skull and skeleton, preserved in articulation, and with little displacement and distortion of the bones. The postcranial skeleton was briefly described by Albert Santa Luca, Cromtpon and Charig in 1976, but its forelimb bones were already mentioned and figured in an article by Peter Galton and Robert T. Bakker in 1974, as the animal was considered significant in establishing the monophyly (natural grouping) of the Dinosaurian class, which was contested by many scientists at the time (including the describers of Heterodontosaurus). The skeleton was fully described in 1980. SAM-PK-K1332 is the most complete heterodontosaurid skeleton described to date. Though a more detailed description of the skull of Heterodontosaurus was long promised, it remained unpublished upon the death of Charig in 1997. It was not until 2011 that the skull was fully described by David B. Norman and colleagues.
Other specimen referred to Heterodontosaurus include the front part of a juvenile skull (SAM-PK-K10487), a fragmentary maxilla (SAM-PK-K1326), a left maxilla with teeth and adjacent bones (SAM-PK-K1334), all collected at the Voyizane locality during expeditions in 1966-1967. A partial snout (NM QR 1788) found in 1975 on Tushielaw Farm south of Voyizane was thought to belong to Massospondylus until 2011. Robert Broom discovered a partial skull, possibly in the Clarens Formation of South Africa, which was sold to the American Museum of Natural History in 1913, as part of a collection that consisted almost entirely of synapsid fossils. The specimen (AMNH 24000) was first identified as belonging to sub adult Heterodontosaurus by Paul Sereno, who reported it in a 2012 monograph about the Heterodontosauridae, the first comprehensive review article about the family. This review also referred a partial postcranial skeleton (SAM-PK-K1328) from Voyizane to Heterodontosaurus, but in 2014, Galton suggested it might belong the related genus Pegomastax instead, which was named by Sereno based on a partial skull from the same locality.
In 1970, Richard A. Thulborn suggested that Heterodontosaurus was a junior synonym of the genus Lycorhinus (named in 1924, also from South Africa), and transferred its type species to the older genus, as the new combination Lycorhinus tucki, which he considered distinct due to slight differences in its teeth and its stratigraphy. He reiterated this claim in 1974, in the description of Lycorhinus consors. In 1974, Charig and Crompton agreed that Heterodontosaurus and Lycorhinus belonged in the same family, Heterodontosauridae, but disagreed that they were similar enough to be considered congeneric. They also pointed out that the fragmentary nature and poor preservation of the Lycorhinus angustidens holotype specimen made it impossible to fully compare it properly to H. tucki. Hopson also defended generic separation pf Heterodontosaurus in 1975, and moved L. consors to its own genus, Abrictosaurus.
Alfred Romer and Oskar Kuhn independently named the family Heterodontosauridae in 1966 as a family of ornithischian dinosaurs including Heterodontosaurus and Lycorhinus. The family was defined as a clade in 1998 by Paul Sereno and redefined by him in 2005 as the stem clade of ornithischia.
The unusual suite of teeth has led to debate over what heterodontosaurs ate. Some scientists think heterodontosaurs were omnivores who used their differently-shaped teeth to eat both plants and small animals.
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- Natural History Museum: "Big teeth for a tiny dinosaur" – three-minute video about Heterodontosaurus presented by Richard Butler