Heterodontosaurus

*Heterodontosaurus tucki*

* Jurassic Park Institute Artwork
* Robert F Walters Artwork

**Heterodontosaurus** (Greek for "different toothed lizard") is a genus of small omnivorous 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* is currently known from specimens of the SAFM (South African Museum) from South Africa. There are two known morphologies of this genus, the second of which is thought by some to represent a different species. The type species, H. tucki, is from the Upper Elliot Formation of the Hettangian age, around 199-196 million years ago.

## History of discovery

The holotype specimen of *Heterodontosaurus tucki* (SAM-PK-K337) was discovered during the British–South African expedition to South Africa and Basutoland (former name of Lesotho) in 1961–1962. Today, it is housed in the Iziko South African Museum. It was excavated on a mountain at an altitude of about 1,890 m (6,201 ft), at a locality called Tyinindini, in the district of Transkei (sometimes referred to as Herschel) in the Cape Province of South Africa. The specimen consists of a crushed but nearly complete skull; associated postcranial remains mentioned in the original description could not be located in 2011. The animal was scientifically described and named in 1962 by palaeontologists Alfred Walter Crompton and Alan J. Charig. The genus name refers to the different-shaped teeth, and the specific name honors George C. Tuck, a director of Austin Motor Company, who supported the expedition. The specimen was not fully prepared by the time of publication, so only the front parts of the skull and lower jaw were 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 ornithischian dinosaurs were known at the time. The preparation of the specimen, i.e. the freeing of the bones from the rock matrix, was very time consuming, since they were covered in a thin, very hard, ferruginous layer containing haematite. This could only be removed by a diamond saw, which damaged the specimen.

In 1966, a second specimen of *Heterodontosaurus* (SAM-PK-K1332) was discovered at the Voyizane locality, in the Elliot Formation of the Stormberg Group of rock formations, 1,770 m (5,807 ft) above sea level, on Krommespruit Mountain. This specimen included both the skull and skeleton, preserved in articulation (i.e. the bones being preserved in their natural position in relation to each other), with little displacement and distortion of the bones. The postcranial skeleton was briefly described by palaeontologists Albert Santa Luca, Crompton and Charig in 1976. Its forelimb bones had previously been discussed and figured in an article by the palaeontologists Peter Galton and Robert T. Bakker in 1974, as the specimen was considered significant in establishing that Dinosauria was a monophyletic natural group, whereas most scientists at the time, including the scientists who described *Heterodontosaurus*, thought that the two main orders Saurischia and Ornithischia were not directly related. 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 the palaeontologist David B. Norman and colleagues.

Other specimens 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 of which were collected at the Voyizane locality during expeditions in 1966–1967, although the first was only identified as belonging to this genus in 2008. A partial snout (NM QR 1788) found in 1975 on Tushielaw Farm south of Voyizane was thought to belong to *Massospondylus* until 2011, when it was reclassified as *Heterodontosaurus*. The palaeontologist 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. This specimen (AMNH 24000) was first identified as belonging to a sub-adult *Heterodontosaurus* by Sereno, who reported it in a 2012 monograph about the Heterodontosauridae, the first comprehensive review article about the family. This review also classified a partial postcranial skeleton (SAM-PK-K1328) from Voyizane as *Heterodontosaurus*. However, in 2014, Galton suggested it might belong to the related genus *Pegomastax* instead, which was named by Sereno based on a partial skull from the same locality. In 2005, a new *Heterodontosaurus* specimen (AM 4766) was found in a streambed near Grahamstown in the Eastern Cape Province; it was very complete, but the rocks around it were too hard to fully remove. The specimen was therefore scanned at the European Synchrotron Radiation Facility in 2016, to help reveal the skeleton, and aid in research of its anatomy and lifestyle, some of which was published in 2021.

In 1970, palaeontologist Richard A. Thulborn suggested that *Heterodontosaurus* was a junior synonym of the genus *Lycorhinus*, which was named in 1924 with the species *L. angustidens*, also from a specimen discovered in South Africa. He reclassified the type species as a member of 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 a third *Lycorhinus* species, *Lycorhinus consors*, after criticism of the synonymy by Galton in 1973. 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*. In spite of the controversy, neither party had examined the *L. angustidens* holotype first hand, but after doing so, palaeontologist James A. Hopson also defended generic separation of *Heterodontosaurus* in 1975, and moved *L. consors* to its own genus, *Abrictosaurus*.

## Description

*Heterodontosaurus* was a small, fleetfooted ornithischian that reached a maximum size of about 3 feet. It had a long, narrow pelvis and a pubis which resembled those possessed by more advanced ornithischians. More unusual was that the hand of *Heterodontosaurus* had five fingers, two of which seem to be opposable. This configuration allowed *Heterodontosaurus* to grasp and manipulate food. The bone in the foot and ankle were fused in a manner reminiscent of those in birds.

### Dentition

Another interesting 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.This bizarre 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.There was even an unlikely opinion now that his fangs had killed and ripped the meat of relatively large animals.

## Classification

When it was described in 1962, *Heterodontosaurus* was classified as a primitive member of Ornithischia, one of the two main orders of Dinosauria (the other being Saurischia). The authors found it most similar to the poorly known genera *Geranosaurus* and *Lycorhinus*, the second of which had been considered a therapsid stem-mammal until then due to its dentition. They noted some similarities with ornithopods, and provisionally placed the new genus in that group. The palaeontologists Alfred Romer and Oskar Kuhn independently named the family Heterodontosauridae in 1966 as a family of ornithischian dinosaurs including *Heterodontosaurus* and *Lycorhinus*. Thulborn instead considered these animals as hypsilophodontids, and not a distinct family. Bakker and Galton recognised *Heterodontosaurus* as important to the evolution of ornithischian dinosaurs, as its hand pattern was shared with primitive saurischians, and therefore was primitive or basal to both groups. This was disputed by some scientists who believed the two groups had instead evolved independently from "thecodontian" archosaur ancestors, and that their similarities were due to convergent evolution. Some authors also suggested a relationship, such as descendant/ancestor, between heterodontosaurids and fabrosaurids, both being primitive ornithischians, as well as to primitive ceratopsians, such as *Psittacosaurus*, though the nature of these relations was debated.

By the 1980s, most researchers considered the heterodontosaurids as a distinct family of primitive ornithischian dinosaurs, but with an uncertain position with respect to other groups within the order. By the early 21st century, the prevailing theories were that the family was the sister group of either the Marginocephalia (which includes pachycephalosaurids and ceratopsians), or the Cerapoda (the former group plus ornithopods), or as one of the most basal radiations of ornithischians, before the split of the Genasauria (which includes the derived ornithischians). Heterodontosauridae was defined as a clade by Sereno in 1998 and 2005, and the group shares skull features such as three or fewer teeth in each premaxilla, caniniform teeth followed by a diastema, and a jugal horn below the eye. In 2006, palaeontologist Xu Xing and colleagues named the clade Heterodontosauriformes, which included Heterodontosauridae and Marginocephalia, since some features earlier only known from heterodontosaurs were also seen in the basal ceratopsian genus *Yinlong*.

Many genera have been referred to Heterodontosauridae since the family was erected, yet *Heterodontosaurus* remains the most completely known genus, and has functioned as the primary reference point for the group in the palaeontological literature. The cladogram below shows the interrelationships within Heterodontosauridae, and follows the analysis by Sereno, 2012:

Heterodontosaurids persisted from the Late Triassic until the Early Cretaceous period, and existed for at least a 100 million years. They are known from Africa, Eurasia, and the Americas, but the majority have been found in southern Africa. Heterodontosaurids appear to have split into two main lineages by the Early Jurassic; one with low-crowned teeth, and one with high-crowned teeth (including *Heterodontosaurus*). The members of these groups are divided biogeographically, with the low-crowned group having been discovered in areas that were once part of Laurasia (northern landmass), and the high-crowned group from areas that were part of Gondwana (southern landmass). In 2012, Sereno labelled members of the latter grouping a distinct subfamily, Heterodontosaurinae. *Heterodontosaurus* appears to be the most derived heterodontosaurine, due to details in its teeth, such as very thin enamel, arranged in an asymmetrical pattern. The unique tooth and jaw features of heterodontosaurines appear to be specialisations for effectively processing plant material, and their level of sophistication is comparable to that of later ornithischians.

In 2017, similarities between the skeletons of *Heterodontosaurus* and the early theropod *Eoraptor* were used by palaeontologist Matthew G. Baron and colleagues to suggest that ornithischians should be grouped with theropods in a group called Ornithoscelida. Traditionally, theropods have been grouped with sauropodomorphs in the group Saurischia. In 2020, palaeontologist Paul-Emile Dieudonné and colleagues suggested that members of Heterodontosauridae were basal marginocephalians not forming their own natural group, instead progressively leading to Pachycephalosauria, and were therefore basal members of that group. This hypothesis would reduce the ghost lineage of pachycephalosaurs and pull back the origins of ornithopods back to the Early Jurassic. The subfamily Heterodontosaurinae was considered a valid clade within Pachycephalosauria, containing *Heterodontosaurus*, *Abrictosaurus*, and *Lycorhinus*.

## Paleobiology

*Heterodontosaurus* is commonly regarded as a herbivorous dinosaur. In 1974, Thulborn proposed that the tusks of the dinosaur played no important role in feeding; rather, that they would have been used in combat with conspecifics, for display, as a visual threat, or for active defence. Similar functions are seen in the enlarged tusks of modern muntjacs and chevrotains, but the curved tusks of warthogs (used for digging) are dissimilar.

Several more recent studies have raised the possibility that the dinosaur was omnivorous and used its tusks for prey killing during an occasional hunt. In 2000, Paul Barrett suggested that the shape of the premaxillary teeth and the fine serration of the tusks are reminiscent of carnivorous animals, hinting at facultative carnivory. In contrast, the muntjac lacks serration on its tusks. In 2008, Butler and colleagues argued that the enlarged tusks formed early in the development of the individual, and therefore could not constitute sexual dimorphism. Combat with conspecifics thus is an unlikely function, as enlarged tusks would be expected only in males if they were a tool for combat. Instead, feeding or defence functions are more likely. It has also been suggested that *Heterodontosaurus* could have used its jugal bosses to deliver blows during combat, and that the palpebral bone could have protected the eyes against such attacks. In 2011, Norman and colleagues drew attention to the arms and hands, which are relatively long and equipped with large, recurved claws. These features, in combination with the long hindlimbs that allowed for fast running, would have made the animal capable of seizing small prey. As an omnivore, *Heterodontosaurus* would have had a significant selection advantage during the dry season when vegetation was scarce.

In 2012, Sereno pointed out several skull and dentition features that suggest a purely or at least preponderantly herbivorous diet. These include the horny beak and the specialised cheek teeth (suitable for cutting off vegetation), as well as fleshy cheeks which would have helped keeping food within the mouth during mastication. The jaw muscles were enlarged, and the jaw joint was set below the level of the teeth. This deep position of the jaw joint would have allowed an evenly spread bite along the tooth row, in contrast to the scissor-like bite seen in carnivorous dinosaurs. Finally, size and position of the tusks are very different in separate members of the Heterodontosauridae; a specific function in feeding thus appears unlikely. Sereno surmised that heterodontosaurids were comparable to today's peccaries, which possess similar tusks and feed on a variety of plant material such as roots, tubers, fruits, seeds and grass. Butler and colleagues suggested that the feeding apparatus of *Heterodontosaurus* was specialised to process tough plant material, and that late-surviving members of the family (*Fruitadens*, *Tianyulong* and *Echinodon*) probably showed a more generalised diet including both plants and invertebrates. *Heterodontosaurus* was characterised by a strong bite at small gape angles, but the later members were adapted to a more rapid bite and wider gapes. A 2016 study of ornithischian jaw mechanics found that the relative bite forces of *Heterodontosaurus* was comparable to that of the more derived *Scelidosaurus*. The study suggested that the tusks could have played a role in feeding by grazing against the lower beak while cropping vegetation.

## Paleoenvironment

*Heterodontosaurus* is known from fossils found in formations of the Karoo Supergroup, including the Upper Elliot Formation and the Clarens Formation, which date to the Hettangian and Sinemurian ages of the Lower Jurassic, around 200–190 million years ago. Originally, *Heterodontosaurus* was thought to be from the Upper Triassic period. The Upper Elliot Formation consists of red/purple mudstone and red/white sandstone, whereas the slightly younger Clarens Formation consists of white/cream-coloured sandstone. The Clarens Formation is less rich in fossils than the Upper Elliot Formation; its sediments also often form cliffs, restricting accessibility for fossil hunters. The Upper Elliot Formation is characterised by animals that appear to be more lightly built than those of the Lower Elliot Formation, which may have been an adaptation to the drier climate at this time in southern Africa. Both formations are famous for their abundant vertebrate fossils, including temnospondyl amphibians, turtles, lepidosaurs, aetosaurs, crocodylomorphs, and non-mammal cynodonts.

Other dinosaurs from these formations include the genasaur *Lesothosaurus*, the basal sauropodomorph *Massospondylus*, and the theropod *Megapnosaurus*. The Lower Elliot Formation shows the largest known heterodontosaurid diversity of any rock unit; besides *Heterodontosaurus*, it contained *Lycorhinus*, *Abrictosaurus*, and *Pegomastax*. Yet another member of the family, *Geranosaurus*, is known from the Clarens Formation. The high heterodontosaurid diversity have led researchers to conclude that different species might have fed on separate food sources in order to avoid competition (niche partitioning). With its highly specialised dentition, *Heterodontosaurus* might have been specialised for tough plant material, while the less specialised *Abrictosaurus* might have predominantly consumed softer vegetation. The position of the individual heterodontosaurid specimens within the rock succession is poorly known, making it difficult to determine how many of these species really were coeval, and which species existed at separate times.

## JPInstitute.com Description

Heterodontosaurus is a small plant-eater that appeared shortly after the beginning of the Jurassic Period. What made it different from the other plant-eaters is that it had more than one kind of tooth. It had small, sharp teeth on the upper portion of the front part of its mouth that looked like small tusks, and kind of a beak on the bottom. Its back teeth were used for cutting up food before it was swallowed. Like the other early dinosaurs, it was speedy and ran on its hind legs.

There is some speculation that the larger canine teeth on this small dinosaur may have been used as a kind of display, possibly for courtship. It is interesting to note that this early dinosaur was found in South Africa, which was close to the South American regions where the Late Triassic early dinosaurs were first discovered. This creature is not thought to have lived in the western hemisphere and may have developed from eastward-migrating early dinosaurs.

## Dinosaur Field Guide Description

Heterodontosaurus ("different-tooth lizard") was a small, fast dinosaur with strong, grasping hands, long arms, and a powerful bite. These features are, for the most part, unnecessary in a plant-eater. It does not take speed, agility, and dexterity to overpower a leaf! Heterodontosaurus's most notable feature, however, is the set of \*fangs" at the front of its mouth. It is quite unusual for a plant-eating dinosaur to have teeth like this, which you'd normally expect in a meat-eater. We know of only one other Early Jurassic group of dinosaurs--the prosauropods--where some of its members had teeth structurally intermediate between those of meat-eaters and of plant-eaters. The same maybe true here. Heterodontosaurus and prosauropods both might be examples of the transition from meat-eater to plant-eater.

### Fun Facts

This small dinosaur could have outrun a ten-year-old human.

### Trivia

A copy of the original skeleton can be seen at the Smithsonian Institution in Washington, D.C.

## Appearance in other media

### *Jurassic Park*

* Remains of the *Heterodontosaurus* was shortly seen in the first Jurassic Park film.

## Links

http://web.archive.org/web/20040214161947fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_heter.html
https://web.archive.org/web/20080723135844/http://kids.yahoo.com/dinosaurs/26--Heterodontosaurus