Heterodontosaurus
Heterodontosaurus is a genus of heterodontosaurid dinosaur that lived during the Early Jurassic, 200–190 million years ago. Its only known member species, Heterodontosaurus tucki, was named in 1962 based on a skull discovered in South Africa. The genus name means "different toothed lizard", in reference to its unusual, heterodont dentition; the specific name honours G. C. Tuck, who supported the discoverers. Further specimens have since been found, including an almost complete skeleton in 1966.
Though it was a small dinosaur, Heterodontosaurus was one of the largest members of its family, reaching between and possibly in length, and weighing between. The skull was elongated, narrow, and triangular when viewed from the side. The front of the jaws were covered in a horny beak. It had three types of teeth; in the upper jaw, small, incisor-like teeth were followed by long, canine-like tusks. A gap divided the tusks from the chisel-like cheek-teeth. The body was short with a long tail. The five-fingered forelimbs were long and relatively robust, whereas the hind-limbs were long, slender, and had four toes.
Heterodontosaurus is the eponymous and best-known member of the family Heterodontosauridae. This family is considered a basal group within the order of ornithischian dinosaurs, while their closest affinities within the group are debated. In spite of the large tusks, Heterodontosaurus is thought to have been herbivorous, or at least omnivorous. Though it was formerly thought to have been capable of quadrupedal locomotion, it is now thought to have been bipedal. Tooth replacement was sporadic and not continuous, unlike its relatives. At least four other heterodontosaurid genera are known from the same geological formations as Heterodontosaurus.
History of discovery
The holotype specimen of Heterodontosaurus tucki was discovered during the British–South African expedition to South Africa and Basutoland in 1961–1962. Today, it is housed in the Iziko South African Museum. It was excavated on a mountain at an altitude of about, at a locality called Tyinindini, in the district of Transkei 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 was discovered at the Voyizane locality, in the Elliot Formation of the Stormberg Group of rock formations, above sea level, on Krommespruit Mountain. This specimen included both the skull and skeleton, preserved in articulation, 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, a fragmentary maxilla, a left maxilla with teeth and adjacent bones, 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 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 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 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 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 dinosaur. The most complete skeleton, SAM-PK-K1332, belonged to an animal measuring about in length. Its weight was variously estimated at,, and in separate studies. The closure of vertebral sutures on the skeleton indicates that the specimen was an adult, and probably fully grown. A second specimen, consisting of an incomplete skull, indicates that Heterodontosaurus could have grown substantially larger – up to a length of and with a body mass of nearly. The reason for the size difference between the two specimens is unclear, and might reflect variability within a single species, sexual dimorphism, or the presence of two separate species. The size of this dinosaur has been compared to that of a turkey. Heterodontosaurus was amongst the largest known members of the family Heterodontosauridae. The family contains some of the smallest known ornithischian dinosaurs – the North American Fruitadens, for example, reached a length of only.Following the description of the related Tianyulong in 2009, which was preserved with hundreds of long, filamentous integuments from neck to tail, Heterodontosaurus has also been depicted with such structures, for example in publications by the palaeontologists Gregory S. Paul and Paul Sereno. Sereno has stated that a heterodontosaur may have looked like a "nimble two-legged porcupine" in life. The restoration published by Sereno also featured a hypothetical display structure located on the snout, above the nasal fossa.
Skull and dentition
The skull of Heterodontosaurus was small but robustly built. The two most complete skulls measured and in length. The skull was elongated, narrow, and triangular when viewed from the side, with the highest point being the sagittal crest, from where the skull sloped down towards the snout tip. The back of the skull ended in a hook-like shape, which was offset to the quadrate bone. The orbit was large and circular, and a large spur-like bone, the palpebral, protruded backwards into the upper part of the opening. Below the eye socket, the jugal bone gave rise to a sideways projecting boss, or horn-like structure. The jugal bone also formed a "blade" that created a slot together with a flange on the pterygoid bone, for guiding the motion of the lower jaw. Ventrally, the antorbital fossa was bounded by a prominent bony ridge, to which the animal's fleshy cheek would have been attached. It has also been suggested that heterodontosaurs and other basal orhithischians had lip-like structures like lizards do, rather than bridging skin between the upper and lower jaws. The proportionally large lower temporal fenestra was egg-shaped and tilted back, and located behind the eye opening. The elliptical upper temporal fenestra was visible only looking at the top of the skull. The left and right upper temporal fenestrae were separated by the sagittal crest, which would have provided lateral attachment surfaces for the jaw musculature in the living animal.The lower jaw tapered towards the front, and the dentary bone was robust. The front of the jaws were covered by a toothless keratinous beak. The upper beak covered the front of the premaxilla bone and the lower beak covered the predentary, which are, respectively, the foremost bones of the upper and lower jaw in ornithischians. This is evidenced by the rough surfaces on these structures. The palate was narrow, and tapered towards the front. The external nostril openings were small, and the upper border of this opening does not seem to have been completely bridged by bone. If not due to breakage, the gap may have been formed by connective tissue instead of bone. The antorbital fossa, a large depression between the eye and nostril openings, contained two smaller openings. A depression above the snout has been termed the "nasal fossa" or "sulcus". A similar fossa is also seen in Tianyulong, Agilisaurus, and Eoraptor, but its function is unknown.
An unusual feature of the skull was the different-shaped teeth for which the genus is named, which is otherwise mainly known from mammals. Most dinosaurs have a single type of tooth in their jaws, but Heterodontosaurus had three. The beaked tip of the snout was toothless, whereas the hind part of the premaxilla in the upper jaw had three teeth on each side. The first two upper teeth were small and cone-shaped, while the third on each side was much enlarged, forming prominent, canine-like tusks. These first teeth were probably partially encased by the upper beak. The first two teeth in the lower jaw also formed canines, but were much bigger than the upper equivalents.
The canines had fine serrations along the back edge, but only the lower ones were serrated at the front. Eleven tall and chisel-like cheek-teeth lined each side of the posterior parts of the upper jaw, which were separated from the canines by a large diastema. The cheek-teeth increased gradually in size, with the middle teeth being largest, and decreased in size after this point. These teeth had a heavy coat of enamel on the inwards side, and were adapted for wear, and they had long roots, firmly embedded in their sockets. The tusks in the lower jaw fit into an indentation within the diastema of the upper jaw. The cheek-teeth in the lower jaw generally matched those in the upper jaw, though the enamel surface of these were on the outwards side. The upper and lower teeth rows were inset, which created a "cheek-recess" also seen in other ornithischians. Despite the different types of teeth, their histology and enamel microstructure was not complex. But while the enamel thinned out towards the outer surface of the teeth, a thick band of wear-resistant dentine arose concurrently with the thinning enamel, and formed the cutting crest of the occlusal surface, a roletypically filled by enamel.