image_caption = Cast of the type specimen of "Heterodontosaurus tucki"
name = Heterodontosaurids
Late Triassic- Cretaceous
familia = Heterodontosauridae
familia_authority = Romer, 1966
subdivision_ranks = Genera
Geranosaurus" " nomen dubium"
"Heterodontosauridae" ("different-toothed lizards") is a family of early
ornithischian dinosaurs that have often been considered basal ornithopods, although recent studies suggest they may have been more closely related to marginocephalians. Although their fossils are rare, they lived around the globe beginning in the Late TriassicPeriod and may have persisted into the Early Cretaceous.
fox-sized dinosaurs less than 2 meters (6.6 ft) in length, including a long tail. They are known mainly for their characteristic teeth, including enlarged canine-like tusks and cheek teeth adapted for chewing, analogous to those of Cretaceous hadrosaurids. Their diet was herbivorous or possibly omnivorous.
Among heterodontosaurids, only "
Heterodontosaurus" itself is known from a complete skeleton. Fragmentary skeletal remains of " Abrictosaurus" are known but have not been fully described, while all other heterodontosaurids are known only from jaw fragments and teeth. Consequently, most heterodontosaurid synapomorphies (defining features) have been described from the teeth and jaw bones.cite book |last=Weishampel |first=David B. |authorlink=David B. Weishampel |coauthors= & Witmer, Lawrence M.|editor=Weishampel, David B.; Dodson, Peter; & Osmólska, Halszka. (eds.). |title=The Dinosauria |year=1990 |publisher=University of California Press |location=Berkeley |isbn=0-520-06727-4 |pages=486-497 |chapter=Heterodontosauridae ] cite book |last= Norman|first=David B. |authorlink=David B. Norman |coauthors=Sues, Hans-Dieter; Witmer, Lawrence M.; & Coria, Rodolfo A. |editor=Weishampel, David B.; Dodson, Peter; & Osmólska, Halszka. (eds.).|title=The Dinosauria |edition= Second Edition |year= 2004|publisher=University of California Press |location=Berkeley |isbn=0-520-24209-2 |pages=393-412 |chapter=Basal Ornithopoda] "Heterodontosaurus" measured just over 1 meter (3.3 ft) in length,cite journal |last=Santa Luca |first=Albert P. |year=1980 |title=The postcranial skeleton of "Heterodontosaurus tucki" (Reptilia, Ornithischia) from the Stormberg of South Africa |journal=Annals of the South African Museum |volume=79 |issue=7 |pages=159–211 ] while the fragmentary remains of " Lycorhinus" indicate an animal up to twice that size.cite journal |last=Gow |first=Christopher E. |year=1990 |title=A tooth-bearing maxilla referable to "Lycorhinus angustidens" Haughton, 1924 (Dinosauria, Ornithischia) |journal=Annals of the South African Museum |volume=99 |issue=10 |pages=367–380]
Both "Abrictosaurus" and "Heterodontosaurus" had very large eyes. Underneath the eyes, the jugal bone projected sideways, a feature also present in
ceratopsians. As in the jaws of most ornithischians, the anterior edge of the premaxilla(a bone at the tip of the upper jaw) was toothless and probably supported a keratinous beak ( rhamphotheca), although heterodontosaurids did have teeth in the posterior section of the premaxilla. A large gap, called a diastema, separated these premaxillary teeth from the those of the maxilla(the main upper jaw bone) in many ornithischians, but this diastema was characteristically arched in heterodontosaurids. The mandible(lower jaw) was tipped by the predentary, a bone unique to ornithischians. This bone also supported a beak similar to the one found on the premaxilla. All the teeth in the lower jaw were found on the dentarybone.
s and is often called the caniniform or 'tusk'. A lower caniniform, larger than the upper, took the first position in the dentary and was accommodated by the arched diastema of the upper jaw when the mouth was closed. These caniniforms were serrated on both the anterior and posterior edges in "Heterodontosaurus" and "Lycorhinus", while those of "Abrictosaurus" bore serrations only on the anterior edge.cite journal |last=Thulborn |first=Richard A. |year=1970 |title=The systematic position of the Triassic ornithischian dinosaur "Lycorhinus angustidens" |journal=Zoological Journal of the Linnean Society |volume=49 |pages=235–245 |doi=10.1111/j.1096-3642.1970.tb00739.x ] cite journal |last=Hopson |first=James A. |year=1975 |title=On the generic separation of the ornithischian dinosaurs "Lycorhinus" and "Heterodontosaurus" from the Stormberg Series (Upper Triassic) of South Africa |journal=South African Journal of Science |volume=71 |pages=302–305 ] In the
Early Cretaceous"Echinodon", there may have been two upper caniniforms, which were on the maxilla rather than the premaxilla,cite book|last=Norman|first=David B. |authorlink=David B. Norman |coauthors=& Barrett, Paul M. |editor=Milner, Andrew; and Batten, David J. (eds.)|chapter=Ornithischian dinosaurs from the Lower Cretaceous (Berriasian) of England|title=Life and Environments in Purbeck Times |series="Special Papers in Palaeontology" 68 |year=2002 |location=London |publisher=Palaeontological Association |pages=161-189 |isbn=0901702730 ] and a North American form from the Late Jurassicmay have had two lower caniniforms on each dentary.cite book|last=Galton|first=Peter M. |authorlink=Peter Galton |editor=Kenneth Carpenter (ed.)|chapter=Teeth of ornithischian dinosaurs (mostly Ornithopoda) from the Morrison Formation (Upper Jurassic) of the western United States. |title=Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs |year=2007 |location=Bloomington |publisher=Indiana University Press |pages=17-47 |isbn=0-253-34817-X]
Like the characteristic tusks, the cheek teeth of
derivedheterodontosaurids were also unique among early ornithischians. Small ridges, or denticles, lined the edges of ornithischian cheek teeth in order to crop vegetation. These denticles extend only a third of the way down the tooth crown from the tip in all heterodontosaurids; in other ornithischians, the denticles extend further down towards the root. Basal forms like "Abrictosaurus" had cheek teeth in both maxilla and dentary that were generally similar to other ornithischians: widely-spaced, each having a low crown and a strongly-developed ridge (cingulum) separating the crown from the root. In more derivedforms like "Lycorhinus" and "Heterodontosaurus", the teeth were chisel-shaped, with much higher crowns and no cingula, so that there was no difference in width between the crowns and the roots.
These derived cheek teeth were overlapping, so that their crowns formed a continuous surface on which food could be chewed. The tooth rows were slightly inset from the side of the mouth, leaving a space outside the teeth that may have been bounded by a muscular
cheek, which would have been necessary for chewing. The much later hadrosaurs and ceratopsians of the Cretaceous Period, as well as many herbivorous mammals, would convergently evolve somewhat analogous dental batteries. As opposed to hadrosaurs, which had thousands of teeth constantly being replaced, tooth replacement in heterodontosaurids occurred far more slowly and several specimens have been found without a single replacement tooth in waiting. Characteristically, heterodontosaurids lacked the small openings (foramina) on the inside of the jaw bones which are thought to have aided in tooth development in most other ornithischians. Heterodontosaurids also boasted a unique spheroidal joint between the dentaries and the predentary, allowing the lower jaws to rotate outwards as the mouth was closed, grinding the cheek teeth against each other. Because of the slow replacement rate, this grinding produced extreme tooth wear that commonly obliterated most of the denticles in older teeth, although the increased height of the crowns gave each tooth a long life.cite book |last=Weishampel |first=David B. |authorlink=David B. Weishampel |year=1984|title=Evolution in jaw mechanics in ornithopod dinosaurs |series="Advances in Anatomy, Embryology, and Cell Biology" 87 |location=Berlin; New York|publisher=Springer-Verlag |pmid=6464809|isbn=0387131140 |issn=0301-5556]
The postcranial anatomy of "Heterodontosaurus tucki" has been well-described, although "H. tucki" is generally considered the most derived of the Early Jurassic heterodontosaurids, so it is impossible to know how many of its features were shared with other species. The forelimbs were long for a dinosaur, over 70% of the length of the hindlimbs. The well-developed deltopectoral crest (a ridge for the attachment of chest and shoulder muscles) of the
humerusand prominent olecranon process (where muscles that extend the forearm were attached) of the ulnaindicate that the forelimb was powerful as well. There were five digits on the manus ('hand'). The first was large, tipped with a sharply curved claw, and would rotate inwards when flexed; Robert Bakkercalled it the 'twist-thumb'.cite book |last=Bakker |first=Robert T. |authorlink=Robert T. Bakker |title=The Dinosaur Heresies: New Theories Unlocking The Mystery of the Dinosaurs and Their Extinction |year=1986 |publisher=William Morrow |location=New York |isbn=0140100555 |pages=453pp.] The second digit was the longest, slightly longer than the third. Both of these digits bore claws, while the clawless fourth and fifth digits were very small and simple in comparison. In the hindlimb, the tibiawas 30% longer than the femur, which is generally considered an adaptation for speed. The tibia and fibulaof the lower leg were fused to the astragalus and calcaneum of the ankle, forming a 'tibiofibiotarsus' convergently with modern birds. Also similarly to birds, the lower tarsal (ankle) bones and metatarsals were fused to form a ' tarsometatarsus.' There are four digits in the pes (hindfoot), with only the second, third, and fourth contacting the ground. The tail, unlike many other ornithischians, did not have ossified tendons to maintain a rigid posture and was probably flexible. The fragmentary skeleton known for "Abrictosaurus" has never been fully described, although the forelimb and manus were smaller than in "Heterodontosaurus". Also, the fourth and fifth digits of the forelimb each bear one fewer phalanx bone.cite journal |last=Thulborn |first=Richard A. |year=1974 |title=A new heterodontosaurid dinosaur (Reptilia: Ornithischia) from the Upper Triassic Red Beds of Lesotho |journal=Zoological Journal of the Linnean Society |volume=55 |pages=151–175 |doi=10.1111/j.1096-3642.1974.tb01591.x ]
Taxonomy and systematics
paleontologist Robert Broomcreated the name " Geranosaurus" in 1911 for dinosaur jaw bones missing all of the teeth.cite journal |last=Broom |first=Robert. |authorlink=Robert Broom |year=1911 |title=On the dinosaurs of the Stormberg, South Africa |journal=Annals of the South African Museum |volume=7 |pages=291–308] In 1924, "Lycorhinus" was named, and classified as a cynodont, by Sidney Haughton.cite journal |last=Haughton |first=Sidney H. |year=1924 |title=The fauna and stratigraphy of the Stormberg Series |journal=Annals of the South African Museum |volume=12 |pages=323–497] "Heterodontosaurus" was named in 1962 and it, "Lycorhinus" and "Geranosaurus" were recognized as closely related ornithischian dinosaurs.cite journal |last=Crompton |first=A.W. |coauthors=& Charig, Alan. |year=1962 |title=A new ornithischian from the Upper Triassic of South Africa |journal=Nature |volume=196 |pages=1074–1077 |doi=10.1038/1961074a0 ] Alfred Romernamed Heterodontosauridae in 1966as a family of ornithischian dinosaurs including "Heterodontosaurus" and "Lycorhinus".cite book |last=Romer |first=Alfred S. |authorlink=Alfred Sherwood Romer |title= Vertebrate Paleontology |edition=Third Edition |year=1966 |publisher= University of Chicago Press |location= Chicago |pages= 468 pp.|isbn=0-7167-1822-7] It was defined as a cladein 1998 by Paul Serenocite journal |last=Sereno |first=Paul C. |authorlink=Paul Sereno |year=1998 |title=A rationale for phylogenetic definitions, with application to the higher-level taxonomy of Dinosauria |journal=Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen |volume=210 |issue=1 |pages=41–83 ] and redefined by him in 2005 as the stem clade consisting of "Heterodontosaurus tucki" and all species more closely related to "Heterodontosaurus" than to " Parasaurolophus walkeri", " Pachycephalosaurus wyomingensis", " Triceratops horridus" or " Ankylosaurus magniventris".cite web |url=http://www.taxonsearch.org/dev/file_home.php |title=Stem Archosauria—TaxonSearch |accessdate=2007-02-24 |last=Sereno |first=Paul C. |authorlink=Paul Sereno |date=2005-11-07 ]
Heterodontosauridae includes the genera "Abrictosaurus", "Lycorhinus" and "Heterodontosaurus", all from South Africa. While
Richard Thulbornonce reassigned all three to "Lycorhinus", all other authors consider the three genera distinct. Within the family, "Heterodontosaurus" and "Lycorhinus" are considered sister taxa, with "Abrictosaurus" as a basal member. "Geranosaurus" is also a heterodontosaurid, but is usually considered a " nomen dubium" because the type specimen is missing all its teeth, making it indistinguishable from any other genus in the family. More recently, the genus "Echinodon" has been considered a heterodontosaurid in several studies. " Lanasaurus" was named for an upper jaw in 1975,cite journal |last=Gow |first=Christopher E. |year=1975 |title=A new heterodontosaurid from the Redbeds of South Africa showing clear evidence of tooth replacement |journal=Zoological Journal of the Linnean Society |volume=57 |pages=335–339 |doi=10.1111/j.1096-3642.1975.tb01895.x ] but more recent discoveries have shown that it belongs to "Lycorhinus" instead, making "Lanasaurus" a junior synonymof that genus. " Dianchungosaurus" was once considered a heterodontosaurid from Asia,cite book |last=Young |first=C.C. |authorlink=Yang Zhongjian |editor=Zhou M. |title= [Collected works of Yang Zhongjian] |year=1982 |publisher=Academica Sinica |location=Beijing |language=Chinese |pages=38–42 |chapter= [A new genus of dinosaur from Lufeng County, Yunnan Province] ] but it has since been shown that the remains were a chimera of prosauropodand mesoeucrocodylian remains.cite journal |last=Barrett |first=Paul M. |coauthors= & Xu Xing. |year=2005 |title=A reassessment of "Dianchungosaurus lufengensis" Yang, 1982a, an enigmatic reptile from the Lower Lufeng Formation (Lower Jurassic) of Yunnan Province, People's Republic of China |journal=Palaeontology |volume=79 |issue=5 |pages=981–986 |doi=10.1666/0022-3360(2005)079 [0981:ARODLY] 2.0.CO;2 |doilabel=10.1666/0022-3360(2005)079[0981:ARODLY]2.0.CO;2] José Bonapartealso classified the South American " Pisanosaurus" as a heterodontosaurid at one time,cite journal |last=Bonaparte |first=Jose F. |authorlink=Jose Bonaparte |year=1976 |title="Pisanosaurus mertii" Casamiquela and the origin of the Ornithischia |journal=Journal of Paleontology |volume=50 |pages=808–820] but this animal is now known to be a more basal ornithischian.cite book |last=Weishampel |first=David B. |authorlink=David B. Weishampel |coauthors= & Witmer, Lawrence M.|editor=Weishampel, David B.; Dodson, Peter; & Osmólska, Halszka. (eds.). |title=The Dinosauria |edition=First Edition |year=1990 |publisher=University of California Press |location=Berkeley |isbn=0-520-06727-4 |pages=416-425 |chapter="Lesothosaurus", "Pisanosaurus", and "Technosaurus"]
Shown above is a cladogram of the Heterodontosauridae, showing relationships between members of the family.
The membership of Heterodontosauridae is well-established in comparison to its uncertain phylogenetic position. Several early studies suggested that heterodontosaurids were very primitive ornithischians. Due to supposed similarities in the morphology of the forelimbs, Robert Bakker proposed a relationship between heterodontosaurids and early
sauropodomorphs like " Anchisaurus", bridging the orders Saurischiaand Ornithischia. The dominant hypothesis over the last several decades has placed heterodontosaurids as basal ornithopods.cite journal |last=Sereno |first=Paul C. |authorlink=Paul Sereno |year=1986 |title=Phylogeny of the bird-hipped dinosaurs |journal=National Geographic Research |volume=2 |issue=2 |pages=234–256] However, others have suggested that heterodontosaurids instead share a common ancestor with Marginocephalia (ceratopsians and pachycephalosaurs),cite journal |last=Zhao Xijin |authorlink=Zhao Xijin |year=1983 |title=Phylogeny and evolutionary stages of Dinosauria |journal=Acta Paleontologica Polonica |volume=28 |issue=1-2 |pages=295–306 ] cite journal |last=Cooper |first= Michael A. |year=1985 |title=A revision of the ornithischian dinosaur "Kangnasaurus coetzeei" Haughton, with a classification of the Ornithischia |journal=Annals of the South African Museum |volume=95 |pages=281–317 ] a hypothesis that has found support in a number of recent studies.cite journal |last=Zhao Xijin |coauthors=Cheng Zhengwu; & Xu Xing. |authorlink=Zhao Xijin |year=1999 |title=The earliest ceratopsian from the Tuchengzi Formation of Liaoning, China |journal=Journal of Vertebrate Paleontology |volume=19 |issue=4 |pages=681–691] cite journal |last=You Hailu |coauthors= Xu Xing; & Wang Xiaolin. |year=2003 |title=A new genus of Psittacosauridae (Dinosauria: Ornithopoda) and the origin and early evolution of marginocephalian dinosaurs |journal=Acta Geologica Sinica (English Edition) |volume=77 |issue=1 |pages=15–20 ] The clade containing heterodontosaurids and marginocephalians has been named Heterodontosauriformes.cite journal |last=Xu Xing |coauthors=Forster, Catherine A.; Clark, James M.; & Mo Jinyou. |authorlink=Xu Xing |year=2006 |title=A basal ceratopsian with transitional features from the Late Jurassic of northwestern China |journal=Proceedings of the Royal Society B: Biological Sciences |volume=273 |pages=2135–2140 |doi=10.1098/rspb.2006.3566 ] Heterodontosaurids have also been seen as basal to both ornithopods and marginocephalians.cite journal |last=Maryanska |first=Teresa |coauthors= & Osmólska, Halszka. |year=1985 |title=On ornithischian phylogeny |journal=Acta Paleontologica Polonica |volume=30 |pages=137–150 ] cite journal |last=Butler |first=Richard J. |year=2005 |title= The 'fabrosaurid' ornithischian dinosaurs of the Upper Elliot Formation (Lower Jurassic) of South Africa and Lesotho. |journal=Zoological Journal of the Linnean Society |volume=145 |issue=2 |pages=175–218 |doi= 10.1111/j.1096-3642.2005.00182.x ] In 2007, a cladisticanalysis suggested that heterodontosaurids are basal to all known ornithischians except "Pisanosaurus", a result that echoes some of the very earliest work on the family.cite journal |last=Butler |first=Richard J. |coauthors=Smith, Roger M.H.; & Norman, David B. |title=A primitive ornithischian dinosaur from the Late Triassic of South Africa, and the early evolution and diversification of Ornithischia |journal=Proceedings of the Royal Society B: Biological Sciences |issue=published online |year=2007 |doi=10.1098/rspb.2007.0367 |volume=274 |pages=2041] cite journal |last=Butler |first=Richard J. |coauthors=Upchurch, Paul; and Norman, David B. |title=The phylogeny of the ornithischian dinosaurs|journal=Journal of Systematic Palaeontology |volume=6 |issue=1 |year=2008 |pages=1–40 |doi=10.1017/S1477201907002271]
While originally known only from the Early Jurassic of southern Africa, heterodontosaurid remains are now known from four
continents. Early in heterodontosaurid history, the supercontinent Pangaeawas still largely intact, allowing the family to achieve a near-worldwide distribution. The oldest known remains are a jaw fragment and isolated teeth from the Laguna Colorada Formationof Argentina, which dates back to the Late Triassic. These remains have a derived morphology similar to "Heterodontosaurus", including a caniniform with serrations on both anterior and posterior edges, as well as high-crowned maxillary teeth lacking a cingulum.cite journal |last=Báez |first=Ana Maria |coauthors=& Marsicano, Claudia A. |year=2001 |title=A heterodontosaurid ornithischian dinosaur from the Upper Triassic of Patagonia. |journal=Ameghiniana |volume=38 | issue =3 |pages=271–279] The most diverse heterodontosaurid faunacomes from the Early Jurassic of southern Africa, where fossils of "Heterodontosaurus", "Abrictosaurus", "Lycorhinus" and the dubious "Geranosaurus" are found. Undescribed Early Jurassic heterodontosaurids are also known from the United Statescite book |last=Sues |first=Hans-Dieter |coauthors=Clark, James M.; & Jenkins, Farish A. |editor=Fraser, Nicholas C.; and Sues, Hans-Dieter (Eds.)|chapter=A review of the Early Jurassic tetrapods from the Glen Canyon Group of the American Southwest |title=In The Shadow of the Dinosaurs: Early Mesozoic Tetrapods |year=1994 |location=Cambridge |publisher=Cambridge University Press |pages=285-294 |isbn=0521458994] and Mexico,cite book |last=Clark |first=James |coauthors=Montellano, Marisol; Hopson, James A., Hernandez, Rene; & Fastovsky, David A. |editor=Fraser, N.C.; and Sues, H.-D. (Eds.)|chapter=An Early or Middle Jurassic tetrapod assemblage from the La Boca Formation, northeastern Mexico |title=In The Shadow of the Dinosaurs: Early Mesozoic Tetrapods |year=1994 |location=Cambridge |publisher=Cambridge University Press |pages=295-302 |isbn=0521458994] respectively. In addition, a great deal of fossil material has been discovered from the Late Jurassic Morrison Formationnear Fruita, Coloradoin the United States. Although it has not been fully described in print, the teeth are similar to the English "Echinodon", while the limb material strongly resembles "Heterodontosaurus", suggesting that this 'Fruita form' is also heterodontosaurid. Heterodontosaurid teeth lacking a cingulum have also been described from Late Jurassic and Early Cretaceous formations in Spainand Portugal.cite journal |last=Sánchez-Hernández |first=Barbara |coauthors=Benton, Michael J.; & Naish, Darren. |year=2007 |title=Dinosaurs and other fossil vertebrates from the Late Jurassic and Early Cretaceous of the Galve area, NE Spain. |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=249 |issue=1-2 |pages=180–215 |doi=10.1016/j.palaeo.2007.01.009] The remains of "Echinodon" have been recently redescribed and may represent a late-surviving heterodontosaurid from the Berriasianstage of the Early Cretaceous in southern England. "Dianchungosaurus" from the Early Jurassic of China is no longer considered a heterodontosaurid; there is no record of the family in Asia.
There exist a number of very bird-like footprint fossils from around the Triassic-Jurassic boundary, found in
Argentinaand South Africa. The ichnogenus"Trisauropodiscus" has been established for some of these; among those footprints some lack a halluximpression, while it is present in others. These fossil predate the oldest known avian theropod, " Archaeopteryx", by up to 55 million years. Though the footprints have not been found associated directly with heterodontosaur remains, these dinosaurs did occur in these locales at the right time. In addition, it is known that heterodontosaurs, due to their convergent foot morphology, must have left very bird-like tracks which, depending on the circumstances and substrate, sometimes had a small hallux impression and sometimes did not.
Most heterodontosaurid fossils are found in
geologic formations that represent arid to semi-arid environments, including the Upper Elliot Formationof South Africaand the Purbeck Bedsof southern England. It has been suggested that heterodontosaurids underwent seasonal aestivation or hibernationduring the driest times of year. Due to the lack of replacement teeth in most heterodontosaurids, it was proposed that the entire set of teeth was replaced during this dormant period, as it seemed that continual and sporadic replacement of teeth would interrupt the function of the tooth row as a single chewing surface. However, this was based on a misunderstanding of heterodontosaurid jaw mechanics, and it has also been shown that heterodontosaurids actually did replace their teeth continually, although more slowly than in other reptiles.cite journal |last=Hopson |first=James A. |year=1980 |title=Tooth function and replacement in early Mesozoic ornithischian dinosaurs: implications for aestivation |journal=Lethaia |volume=13 |issue=1 |pages=93–105 |doi=10.1111/j.1502-3931.1980.tb01035.x ] There is currently no evidence that supports the hypothesisof aestivation in heterodontosaurids.
While the cheek teeth of heterodontosaurids are clearly adapted for grinding tough plant material, their diet may have been omnivorous. The pointed premaxillary teeth and sharp, curved claws on the forelimbs suggest some degree of predatory behavior. It has been suggested that the long, powerful forelimbs of "Heterodontosaurus" may have been useful for tearing into insect nests, similarly to modern
anteaters. These forelimbs may have also functioned as digging tools, perhaps for roots and tubers. The length of the forelimb compared to the hindlimb suggests that "Heterodontosaurus" might have been partially quadrupedal, and the prominent olecranon process and hyperextendable digits of the forelimb are found in many quadrupeds. However, the manus is clearly designed for grasping, not weight support. Many features of the hindlimb, including the long tibia and foot, as well as the fusion of the tibiofibiotarsus and tarsometatarsus, indicate that heterodontosaurids were adapted to run quickly on the hindlegs, so it is unlikely that "Heterodontosaurus" moved on all four limbs except perhaps when feeding.
The short tusks found in all known heterodontosaurids strongly resemble tusks found in modern
musk deer, peccaries and pigs. In many of these animals (as well as the longer-tusked walrusand Asian elephants), this is a sexually dimorphic trait, with tusks only found in males. The type specimenof "Abrictosaurus" lacks tusks and was originally described as a female. While this remains possible, the unfused sacral vertebrae and short face indicate that this specimen represents a juvenile animal, while a second, larger specimen of "Abrictosaurus" clearly possesses tusks. Therefore, it is possible that tusks are found only in adults, rather than being a secondary sexual characteristic of males. These tusks could have been used for combat or display with members of the same species or with other species.
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