Heterodontosaurus
| Heterodontosaurus |
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| Cast of SAM-PK-K1332, University of California Museum of Paleontology | |
Scientific classification  |
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| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Order: | †Ornithischia |
| Family: | †Heterodontosauridae |
| Subfamily: | †Heterodontosaurinae |
| Genus: | †Heterodontosaurus Crompton & Charig, 1962 |
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Heterodontosaurus is a genus of heterodontosaurid dinosaur that lived during the Hettangian age of the Early Jurassic period, 199–196 million years ago. The holotype specimen (a skull) was discovered in South Africa, and the animal was named H. tucki in 1962. The genus name means "different toothed lizard", in reference to its unusual dentition, and the specific name honours G. C. Tuck, who assisted the discoverers. Further specimens have since been found, including an almost complete individual in 1966.
Heterodontosaurus was a small dinosaur, and reached between 1.18 and possibly 1.75 m in length, and between 2 and 10 kg in weight. It had prominent canine teeth, and is thought to have eaten plants, despite its canines. Heterodontosaurus is a member of the heterodontosauridae family, of which it is the most completely known member. This family is considered one of the most primitive or basal groups within the order of ornithischian dinosaurs. At least four other heterodontosaur genera are known from the same geological formations as Heterodontosaurus.
Contents
Description
Heterodontosaurus was a small dinosaur. The most complete skeleton, SAM-PK-1332, belonged to an individual measuring about 1.18 m in length. Its weight was estimated at 1.8 kg,[1] 2.59 kg, and 3.4 kg in separate studies.[2] Closure of vertebral sutures indicates it is was adult and probably fully grown individual. An incomplete skull indicates that Heterodontosaurus could grow substantially larger – up to a length of 1.75 m and a body mass of nearly 10 kg. The reason for the size difference between the two specimens is unclear, and might reflect variability within the species, sexual dimorphism, or the presence of two separate species.[2][3] Heterodontosaurus was amongst the largest known members of the Heterodontosauridae family.[4] The family contains some of the smallest known ornithischians[5] – the North American Fruitadens, for example, reached a length of only 65 to 75 cm.[6]
The neck consisted of nine cervical vertebrae, which would have formed 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.[5][7] The trunk was short, consisting of 12 dorsal and 6 fused sacral vertebrae.[5] 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 ornithischian dinosaurs 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 was therefore probably flexible.[7] It had a long, narrow pelvis and a pubis which resembled those possessed by more advanced ornithischians.[8][9][10]
The forelimbs were robustly built[4] 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.[5] The hand was large, approaching the humerus in length, and possessed five fingers equipped for grasping.[5][4] The bones in the foot and ankle were fused in a manner reminiscent of those in birds.[8][10] The second finger was the longest, followed by the third and the first finger (the thumb).[5] The first three fingers ended in large and strong claws. The fourth and fifth fingers were strongly reduced and possibly vestigial (functionless). The phalangeal formula, which states the number of finger bones in each finger starting from the first, was 2-3-4-3-2.[5]
The hindlimbs were long and gracile and ended in four toes, the first of which (the hallux) did not contact 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.[5] This constellation can also be found in modern birds, where it has evolved independently.[4] The tibiotarsus was about 30% longer than the femur.[5]
Following the discovery of the related Tianyulong in 2009, which was preserved with long, filamentous integuments (comparable to feathers or hair), Heterodontosaurus has also been depicted with such structures, for example in publications by Gregory S. Paul and Paul Sereno.[2] 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.[4]
Skull and dentition
The skull of Heterodontosaurus was robustly built and triangular in side view. The front of the jaws where covered by a toothless keratinous (covered in horn) 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. 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 above the eye opening.[9] Below the eye socket, the jugal bone gave rise to a sidewards projecting horn-like structure. The antorbital fossa, a large depression between the eye and nostril openings, contained two smaller openings.[5] Ventrally, the antorbital fossa was bounded by a prominent bony ridge, to which the animal's fleshy cheek would have been attached. 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 in top view of the skull. The left and right upper temporal fenestrae were separated by a pronounced sagittal crest, which would have provided attachment sides for the jaw musculature in the living animal.[9]
The most unusual feature of the skull was the eponymous differentiation of the teeth (heterodonty), which is otherwise mainly known from mammals. Most dinosaurs (and indeed most reptiles) have a single type of tooth in their jaws, while Heterodontosaurus had three. The beaked tip of the snout was toothless, and only the hind part of the premaxilla in the upper jaw had three teeth on each side. The first two upper teeth where small and pen shaped, while the third on each side was much enlarged, forming prominent canines or tusks. 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. Tall and chisel-like teeth lined the posterior parts of the upper and lower jaws, which were separated from the canines of the upper jaw by a large diastema (gap). The tusks in the lower jaw fit into an indentation within the diastema of the upper jaw.[9][5]
Discovery
The holotype specimen of Heterodontosaurus tucki (SAM-PK-K1337, housed in the Iziko 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).[4][11] 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.[9]
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).[12][13] The skeleton was fully described in 1980.[7] SAM-PK-K1332 is the most complete heterodontosaurid skeleton described to date.[2] Though a more detailed description of the skull of Heterodontosaurus was long promised, it remained unpublished upon the death of Charig in 1997.[14] It was not until 2011 that the skull was fully described by David B. Norman and colleagues.[9]
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, though 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. 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.[4][15] 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.[2]
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.[16] He reiterated this claim in 1974, in the description of Lycorhinus consors, after criticism of the synonymy by Galton in 1973.[17] 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.[18] In spite of the controversy, neither party had examined the L. angustidens holotype first hand, but after doing so, James A. Hopson also defended generic separation of Heterodontosaurus in 1975, and moved L. consors to its own genus, Abrictosaurus.[19]
Classification
In the 1962 description, Heterodontosaurus was classified as a primitive ornithischian, one of the two main orders of Dinosauria (the other being saurischia), and the authors found it most similar to the poorly known genera Geranosaurus and Lycorhinus (the latter 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.[11] Alfred Romer and Oskar Kuhn independently named the family Heterodontosauridae in 1966 as a family of ornithischian dinosaurs including Heterodontosaurus and Lycorhinus.[4][20][21] Thulborn instead considered these animals as hypsilophodontids, and not a distinct family.[16] Bakker and Galton recognised Heterodontosaurus as important to the evolution of ornithischian dinosaurs, as its hand pattern was shared with primitive saurischians, and therefore primitive (or basal) to both groups.[13] 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 between heterodontosaurids and fabrosaurids (including as descendant/ancestor), both being primitive ornithiscians, as well as to primitive ceratopsians, such as Psittacosaurus, though the nature of these relations were debated.[7]
By the 1980s, most researchers considered the heterodontosaurids as a distinct family of primitive ornithischian dinosaurs, but of unclear position in regard to other groups within the order. By the early 21th century, the prevailing theories were that the family was the sister group of either Marginocephalia (which includes pachycephalosaurids and ceratopsians) or cerapoda (the former group plus ornithopods), or as one of the most basal radiations of ornithischians, before the split of genasauria (which includes the derived ornithischians).[9] Heterodontosauridae was defined as a clade by Paul 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.[22] 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.[4][2]
The cladogram below shows the interrelationships within Heterodontosauridae, and follows the analysis by Galton, 2014:[2]
| Heterodontosauridae |
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The Heterodontosauridae family persisted from the Late Triassic until the Early Cretaceous period, and existed for at least a 100 million years. Heterodontosaurids 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 deep-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 deep-crowned group from areas that were part of Gondwana (Southern landmass). In 2012, Sereno labelled most members of the latter grouping a unique subfamily, Heterodontosaurinae. Heterodontosaurus appears to be the most derived (or advanced) 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 ornithiscians.[4]
Palaeobiology
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.[8][23]
Growth and ontogeny
The ontogeny, the development of the individual from juvenile to adult, is poorly known, as juvenile specimens are scarce. As shown by a juvenile skull described in 2006 (SAM-PK-K10487), the eye sockets got proportionally smaller as the animal grows, while the snout became longer and comprised additional teeth. Similar changes have been reported for several other dinosaurs. The morphology of the teeth, however, did not change during ontogeny, indicating that juveniles had the same diet as adults. The length of the juvenile skull is reconstructed at 45 mm. Assuming similar body proportions as adult individuals, the body length of this juvenile would have been 45 cm. Indeed, the individual probably would have been smaller, since juvenile animals in general show proportionally larger heads.[15]
Sexual dimorphism
Richard Thulborn in 1974 suggested the large tusks of heterodontosaurids to represent a secondary sex characteristic. According to this author, only adult male individuals would have possessed fully developed tusks; the holotype specimen of the related Abrictosaurus, which lacked tusks altogether, would have represented a female.[17] This hypothesis was questioned by Richard Butler and colleagues in 2006, arguing that the newly described juvenile skull (SAM-PK-K10487) showed tusks despite its early developmental state. At this state, secondary sex characteristics cannot be expected. Furthermore, tusks are present in almost all known skulls; the presence of a sexual dimorphism however would suggest a 50:50 ratio between individuals bearing tusks and those lacking tusks. The only exception is the holotype specimen of Abrictosaurus – the lack of tusks in this individual is interpreted as a specialization of this particular genus.[15]
Tooth replacement and aestivation
Much controversy revolved around the question whether or not, or to which degree, Heterodontosaurus showed the continuous tooth replacement that is typical for other dinosaurs and reptiles. In 1974 and 1978, Thulborn found that the skulls known at that time lack any indications of a continuous tooth replacement: The cheek teeth of the known skulls are worn uniformly, indicating that they formed simultaneously. Newly erupted teeth are absent. Further evidence was derived from the wear facets of the teeth, which were formed due to tooth-to-tooth contact of the lower with the upper dentition. The wear facets were merged into one another, forming a continuous surface along the complete tooth row. This surface indicates that food procession was achieved by forwards/backwards movements of the jaws, not by simple vertical movements which were present in related dinosaurs such as Fabrosaurus. Forwards/backwards movements are only possible if the teeth are worn uniformly, again strengthening the case for the lack of a continuous tooth replacement. Simultaneously, however, Thulborn stressed that a regular tooth replacement was essential for these animals, as the supposed diet consisting of tough plant material would have led to a fast abrasion of the teeth. These observations led Thulborn to conclude that Heterodontosaurus must have exchanged its whole dentition at once on a regular basis. Such a complete replacement could only have been possible within phases of aestivation, when the animal did not fed. Aestivation also complies with the supposed habitat of the animals, which would have been desert-like, including hot dry seasons when food was scarce.[17][24][25]
A comprehensive analysis conducted by Hopson questioned Thulborn's ideas in 1980. Hopson showed that the wear facet patterns on the teeth in fact indicate vertical and lateral rather than forwards/backwards jaw movements. Furthermore, he demonstrated variability in the degree of tooth wear, indicating continuous tooth replacement. However, he acknowledged that X-ray images of the most complete specimen showed that this individual indeed lacked unerupted replacement teeth. According to Hopson, this indicates that only juveniles possessed a continuous tooth replacement, which then ceased in adults. Thulborn's aestivation hypothesis was rejected by Hopson due to a lack of evidence.[25]
Richard Butler and colleagues conducted computer tomography scans of the juvenile skull SAM-PK-K10487. To the surprise of these researchers, no yet unerupted replacement teeth could be found even in this early ontogenetic stage. Despite these findings, the authors argued that tooth replacement must have been present because the juvenile displayed the same tooth morphology than adult individuals – morphology would have changed if the tooth would simply have grown continuously. In conclusion, Butler and colleagues suggested that tooth replacement in Heterodontosaurus must have been more sporadically than in related dinosaurs.[15] Unerupted replacement teeth in Heterodontosaurus were not found until 2011, when Norman and colleagues described the upper jaw SAM-PK-K1334. Another juvenile skull (AMNH 24000) described by Paul Sereno in 2012 also yielded unerupted replacement teeth. As shown by these discoveries, tooth replacement in Heterodontosaurus was episodically and not continuous as in other heterodontosaurids. The unerupted teeth are triangular in lateral view, which is the typical tooth morphology in basal ornithischians. The characteristic chisel-like shape of the fully erupted teeth therefore resulted from tooth-to-tooth contact of the lower with the upper dentition.[4][9]
Locomotion and metabolism
While most studies depict Heterodontosaurus as a bipedal (two-legged) runner, some propose a partial or fully quadrupedal (four-legged) locomotion. In 1980, Santa Luca described several features of the forelimb that are also present in recent quadrupedal animals and imply a strong arm musculature: These include a large olecranon (a bony eminence forming the uppermost part of the ulna), enlarging the lever arm of the forearm. The medial epicondyle of the humerus was enlarged, providing attachment sites for strong flexor muscles of the forearm. Furthermore, projections on the claws might have increased forward thrust of the hand during walking. According to Santa Luca, Heterodontosaurus was quadrupedal when moving slowly but was able to switch to a much faster, bipedal run.[7] Polish paleontologists Teresa Maryańska and Halszka Osmólska in 1985 supported Santa Luca's hypothesis; as an additional argument, they noted that the dorsal spine was strongly flexed downwards in the most completely known specimen.[26] In 1987, Gregory Paul suggested that Heterodontosaurus was obligatory quadrupedal – for fast locomotion, these animals would have galloped. However, Paul did not cite any anatomical features which may support this hypothesis.[27] David Weishampel and Lawrence Witmer in 1990 as well as Norman and colleagues in 2004 argued in favor of an exclusively bipedal locomotion, based on the morphology of the claws and shoulder girdle.[5][28] The anatomical evidence suggested by Santa Luca was identified as adaptations for foraging; the robust and strong arms might have been used for digging up roots and breaking open insect nests.[5]
Most studies consider dinosaurs as endotherm (warm-blooded) animals, with an elevated metabolism comparable to that of today's mammals and birds. In a 2009 study, Herman Pontzer and colleagues calculate the aerobic endurance of various dinosaurs. Even at moderate running speeds, Heterodontosaurus would have exceeded the maximum aerobic capabilities that would have been available for an ectotherm (cold-blooded) animal, indicating endothermy in this genus.[29]
Palaeoecology
Heterodontosaurus is known from formations in the Upper Stormberg Group, including the Upper Elliot Formation and the Clarens Formation, which date to the Hettangian age of the Lower Jurassic, around 199-196 million years ago (Heterodontosaurus was originally thought to be Upper Triassic in age). The Elliot Formation consists of red/purple mudstone and red/white sandstone, whereas the Clarens Formation consists of white/cream-coloured sandstone. The animals of the Upper Elliot Formation appear to be more lightly build than those of the Lower Elliot Formation, and adapted to the drier climate at this time in southern Africa. Other heterodontosaurids from the Stormberg Group include Lycorhinus, Abrictosaurus, and Pegomastax from the Elliot Formation, and Geranosaurus from the Clarens Formation. Other dinosaurs from these formations include the genasaur Lesothosaurus, the sauropodomorph Massospondylus, and the theropod Megapnosaurus. Other tetrapod animals include temnospondyl amphibians, turtles, lepidosaurs, aetosaurs, crocodylomorphs, and non-mammal cynodonts. Tracks of various animals, including pterosaurs, have also been identified.[11][30][4]
References
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- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 7.0 7.1 7.2 7.3 7.4 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 8.0 8.1 8.2 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 10.0 10.1 "Heterodontosaurus." In: Dodson, Peter & Britt, Brooks & Carpenter, Kenneth & Forster, Catherine A. & Gillette, David D. & Norell, Mark A. & Olshevsky, George & Parrish, J. Michael & Weishampel, David B. The Age of Dinosaurs. Publications International, LTD. p. 37. ISBN 0-7853-0443-6.
- ↑ 11.0 11.1 11.2 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 13.0 13.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 15.0 15.1 15.2 15.3 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 16.0 16.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 17.0 17.1 17.2 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Kuhn, O. (1966) Die Reptilien. Verlag Oeben, Krailling near Munich, 154 pp.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Tiny Juvenile Dinosaur Fossil Sheds Light on Evolution of Plant Eaters Newswise, Retrieved on October 23, 2008.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 25.0 25.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ Lua error in package.lua at line 80: module 'strict' not found.
External links
- Natural History Museum: "Big teeth for a tiny dinosaur" – three-minute video about Heterodontosaurus presented by Richard Butler
