| Heterodontosaurids | |
|---|---|
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| Cast of specimen SAM-PK-K1332 ofHeterodontosaurus tucki, University of California Museum of Palaeontology | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Saphornithischia |
| Family: | †Heterodontosauridae Romer, 1966 (Kuhn,1966) |
| Subgroups[1] | |
Heterodontosauridae is afamily ofornithischiandinosaurs that were likely among the mostbasal (primitive) members of the group. Theirphylogenetic placement is uncertain but they are most commonly found outside of the groupGenasauria.[2] Although theirfossils are relatively rare and their group small in numbers, they have been found on all continents exceptAustralia andAntarctica, with a range spanning the Early Jurassic to theEarly Cretaceous.
Heterodontosaurids werefox-sized dinosaurs less than 2 metres (6.6 feet) in length, including a long tail. They are known mainly for their characteristic teeth, including enlargedcanine-like tusks and cheek teeth adapted for chewing,analogous to those of Cretaceoushadrosaurids. Their diet washerbivorous or possiblyomnivorous.
Among heterodontosaurids, onlyHeterodontosaurus itself is known from a complete skeleton. Fragmentary skeletal remains ofAbrictosaurus are known but have not been fully described, while most other heterodontosaurids are known only from jaw fragments and teeth. Consequently, most heterodontosauridsynapomorphies (defining features) have been described from the teeth and jaw bones.[3][4]Heterodontosaurus measured just over 1 meter (3.3 ft) in length,[5] while the fragmentary remains ofLycorhinus may indicate a larger individual.[6]
Tianyulong from China appears to preserve filamentous integument which has been interpreted to be a variant of the proto-feathers found in some theropods. These filaments include a crest along its tail. The presence of this filamentous integument has been used to suggest that both ornithischians and saurischians wereendothermic.[7]
BothAbrictosaurus andHeterodontosaurus had very largeeyes. Underneath the eyes, thejugal bone projected sideways, a feature also present inceratopsians. As in the jaws of most ornithischians, theanterior edge of thepremaxilla (a bone at the tip of the upper jaw) was toothless and probably supported akeratinous beak (rhamphotheca), although heterodontosaurids did have teeth in theposterior section of the premaxilla. A large gap, called adiastema, separated these premaxillary teeth from those of themaxilla (the main upper jaw bone) in many ornithischians, but this diastema was characteristically arched in heterodontosaurids. Themandible (lower jaw) was tipped by thepredentary, a bone unique to ornithischians. This bone also supported a beak similar to the one found on thepremaxilla. All the teeth in the lower jaw were found on thedentary bone.[3]
Heterodontosaurids are named for their stronglyheterodontdentition. There were three premaxillary teeth. In theEarly JurassicAbrictosaurus,Heterodontosaurus, andLycorhinus, the first two premaxillary teeth were small and conical, while the much larger third tooth resembled the canines ofcarnivoranmammals 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.[3] These caniniforms were serrated on both the anterior and posterior edges inHeterodontosaurus andLycorhinus, while those ofAbrictosaurus bore serrations only on the anterior edge.[8][9] In theEarly CretaceousEchinodon, there may have been two upper caniniforms, which were on the maxilla rather than the premaxilla,[10] andFruitadens from theLate Jurassic may have had two lower caniniforms on each dentary.[11][12]
Like the characteristic tusks, the cheek teeth ofderived heterodontosaurids 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 likeAbrictosaurus 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 morederived forms likeLycorhinus andHeterodontosaurus, the teeth were chisel-shaped, with muchhigher crowns and no cingula, so that there was no difference in width between the crowns and the roots.[3]
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 muscularcheek, which would have been necessary for chewing. The hadrosaurs and ceratopsians of the Cretaceous Period, as well as many herbivorous mammals, wouldconvergentlyevolve somewhat analogousdental batteries. As opposed to hadrosaurs, which had hundreds 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 uniquespheroidal 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.[13]
The postcranial anatomy ofHeterodontosaurus tucki has been well-described, althoughH. 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.[3] 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 ofchest andshoulder muscles) of thehumerus and prominentolecranon process (wheremuscles that extend the forearm were attached) of theulna indicate that the forelimb was powerful as well. There were five digits on themanus ('hand'). The first was large, tipped with a sharply curved claw, and would rotate inwards when flexed;Robert Bakker called it the 'twist-thumb'.[14] 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, thetibia was 30% longer than thefemur, which is generally considered an adaptation for speed. The tibia andfibula of the lower leg were fused to theastragalus andcalcaneum of the ankle, forming a 'tibiofibiotarsus' convergently with modernbirds. Also similarly to birds, the lowertarsal (ankle) bones andmetatarsals were fused to form a 'tarsometatarsus.' There are four digits in thepes (hindfoot), with only the second, third, and fourth contacting the ground. The tail, unlike many other ornithischians, did not haveossifiedtendons to maintain a rigid posture and was probably flexible.[5] The fragmentary skeleton known forAbrictosaurus has never been fully described, although the forelimb and manus were smaller than inHeterodontosaurus. Also, the fourth and fifth digits of the forelimb each bear one fewerphalanx bone.[15]
South AfricanpaleontologistRobert Broom created the nameGeranosaurus in 1911 for dinosaur jaw bones missing all of the teeth and some partial associated limb bones.[16] In 1924,Lycorhinus was named, and classified as acynodont, bySidney Haughton.[17]Heterodontosaurus was named in 1962 and it,Lycorhinus andGeranosaurus were recognized as closely related ornithischian dinosaurs.[18]Alfred Romer named Heterodontosauridae in 1966 as a family of ornithischian dinosaurs includingHeterodontosaurus andLycorhinus.[19] Kuhn independently proposed Heterodontosauridae in the same year and is sometimes cited as its principal author.[20] It was defined as aclade in 1998 byPaul Sereno[21] and redefined by him in 2005 as the stem clade consisting ofHeterodontosaurus tucki and all species more closely related toHeterodontosaurus than toParasaurolophus walkeri,Pachycephalosaurus wyomingensis,Triceratops horridus, orAnkylosaurus magniventris.[22] Heterodontosauridae was given a formal definition in thePhyloCode by Daniel Madzia and colleagues in 2021 as "the largest clade containingHeterodontosaurus tucki, but notIguanodon bernissartensis,Pachycephalosaurus wyomingensis,Stegosaurus stenops, andTriceratops horridus".[2] Heterodontosaurinae is astem-based taxon defined phylogenetically for the first time byPaul Sereno in 2012 as "the most inclusiveclade containingHeterodontosaurus tucki but notTianyulong confuciusi,Fruitadens haagarorum,Echinodon becklesii."[1]
Heterodontosauridae includes the generaAbrictosaurus,Lycorhinus, andHeterodontosaurus, all from South Africa. WhileRichard Thulborn once reassigned all three toLycorhinus,[15] all other authors consider the three genera distinct.[9] Within the family,Heterodontosaurus andLycorhinus are consideredsister taxa, withAbrictosaurus as a basal member.[4]Geranosaurus is also a heterodontosaurid, but is usually considered anomen dubium because the type specimen is missing all its teeth, making it indistinguishable from any other genus in the family.[3] More recently, the genusEchinodon has been considered a heterodontosaurid in several studies.[10][11]Lanasaurus was named for an upper jaw in 1975,[23] but more recent discoveries have shown that it belongs toLycorhinus instead, makingLanasaurus ajunior synonym of that genus.[6]Dianchungosaurus was once considered a heterodontosaurid fromAsia,[24] but it has since been shown that the remains were achimera ofprosauropod andmesoeucrocodylian remains.[25]José Bonaparte also classified the South AmericanPisanosaurus as a heterodontosaurid at one time,[26] but this animal is now known to be a more basal ornithischian.[27]
The membership of Heterodontosauridae is well-established in comparison to its uncertainphylogenetic position. Several early studies suggested that heterodontosaurids were very primitive ornithischians.[5][18] Due to supposed similarities in the morphology of the forelimbs, Robert Bakker proposed a relationship between heterodontosaurids and earlysauropodomorphs likeAnchisaurus, bridging the ordersSaurischia andOrnithischia.[14] The dominant hypothesis over the last several decades has placed heterodontosaurids as basal ornithopods.[3][4][8][28] However, others have suggested that heterodontosaurids instead share a common ancestor withMarginocephalia (ceratopsians andpachycephalosaurs),[29][30] a hypothesis that has found support in some early 21st century studies.[31][32] The clade containing heterodontosaurids and marginocephalians has been namedHeterodontosauriformes.[33] Heterodontosaurids have also been seen as basal to both ornithopods and marginocephalians.[34][35] In 2007, acladistic analysis suggested that heterodontosaurids are basal to all known ornithischians exceptPisanosaurus, a result that echoes some of the very earliest work on the family.[36][37] However, a study by Bonaparte found the Pisanosauridae to be synonymous with the Heterodontosauridae and not a separate family in its own right, thereby includingPisanosaurus as a heterodontosaur.[38] Butler et al. (2010) found the Heterodontosauridae to be the most basal known significant ornithischian radiation.[39]
Thecladogram below shows the interrelationships within Heterodontosauridae, and follows the analysis by Sereno, 2012:[40]
| Heterodontosauridae | |
A 2020 reworking ofCerapoda by Dieudonné and colleagues recovered the animals traditionally considered 'heterodontosaurids' as a basal grouping within Pachycephalosauria, paraphyletic with respect to the traditional, dome-headed pachycephalosaurs. This result was based on numerous skull characteristics including thedentition, and also to account for the fact thatpachycephalosaur fossils are completely unknown from theJurassic period. Modern understanding of ornithischian phylogeny implies that Jurassic pachycephalosaurs must exist, because numerous Jurassicceratopsians have been found, yet no such pachycephalosaurs have been confidently identified. This analysis was done to elaborate on the findings of Baron and colleagues (2017), which foundChilesaurus to be a basalornithischian.[41] The phylogenetic analysis was conducted withChilesaurus coded as an ornithischian, which also had implications for the phylogeny ofornithopods.
The cladogram below is an abridged version of Dieudonne and colleagues' findings:[42]
| Ornithischia |
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While originally known only from the Early Jurassic of southern Africa, heterodontosaurid remains are now known from fourcontinents. Early in heterodontosaurid history, thesupercontinentPangaea was still largely intact, allowing the family to achieve a near-worldwidedistribution.[10] The oldest known possible heterodontosaurid remains are a jaw fragment and isolated teeth from theLaguna Colorada Formation ofArgentina, which dates back to theLate Triassic. These remains have a derived morphology similar toHeterodontosaurus, including a caniniform with serrations on both anterior and posterior edges, as well as high-crowned maxillary teeth lacking a cingulum.[43] Irmiset al. (2007) tentatively agreed that this fossil material represents a heterodontosaurid, but stated that additional material is needed to confirm this assignment because the specimen is poorly preserved,[44] while Sereno (2012) only stated that this material may represent an ornithischian or even specifically a heterodontosaurid.[1]Olsen, Kent & Whiteside (2010) noted that the age of the Laguna Colorada Formation itself is poorly constrained, and thus it wasn't conclusively determined whether the putative heterodontosaurid from this formation is of Triassic or Jurassic age.[45] The most diverse heterodontosauridfauna comes from the Early Jurassic of southern Africa, where fossils ofHeterodontosaurus,Abrictosaurus,Lycorhinus, and the dubiousGeranosaurus are found.[3]
Undescribed Early Jurassic heterodontosaurids are also known from theUnited States[46] andMexico,[47] respectively. In addition, beginning in the 1970s, a great deal of fossil material was discovered from the Late JurassicMorrison Formation nearFruita,Colorado in the United States.[11] Described in print in 2009, this material was placed in the genusFruitadens.[12] Indeterminate heterodontosaurid teeth have been reported from theMiddle Jurassic (Bathonian)Great Oolite Group of Great Britain,[48] as well as the equivalently agedItat Formation ofWestern Siberia.[49] Heterodontosaurid teeth lacking a cingulum have also been described from Late Jurassic and Early Cretaceous formations inSpain andPortugal.[50] The remains ofEchinodon were redescribed in 2002, showing that it may represent a late-surviving heterodontosaurid from theBerriasian stage of the Early Cretaceous in southern England.[10]Dianchungosaurus from the Early Jurassic of China is no longer considered a heterodontosaurid; though one Middle-Late Jurassic Asian form is known (Tianyulong).[7] Indeterminate cheek teeth possibly representing heterodontosaurids are also known from theBarremian agedWessex Formation of southern England, which if confirmed would represent the youngest record of the group.[51]
Most heterodontosaurid fossils are found ingeologic formations that represent arid to semi-arid environments, including theUpper Elliot Formation ofSouth Africa and thePurbeck Beds of southern England.[4] It has been suggested that heterodontosaurids underwent seasonalaestivation orhibernation during 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.[15] However, this was based on a misunderstanding of heterodontosaurid jaw mechanics.[52] It was thought that heterodontosaurids actually did replace their teeth continually, though more slowly than in otherreptiles, butCT scanning of skulls from juvenile and matureHeterodontosaurus shows no replacement teeth.[53] There is currently no evidence that supports thehypothesis of aestivation in heterodontosaurids,[3] but it cannot be rejected, based on the skull scans.[53]
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 ofHeterodontosaurus may have been useful for tearing into insect nests, similarly to modernanteaters. These forelimbs may have also functioned as digging tools, perhaps forroots andtubers.[3]
The length of the forelimb compared to the hindlimb suggests thatHeterodontosaurus might have been partiallyquadrupedal, 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 thatHeterodontosaurus moved on all four limbs except perhaps when feeding.[5]
The short tusks found in all known heterodontosaurids strongly resemble tusks found in modernmusk deer,peccaries andpigs. In many of these animals (as well as the longer-tuskedwalrus andAsian elephants), this is asexually dimorphic trait, with tusks only found in males. Thetype specimen ofAbrictosaurus lacks tusks and was originally described as a female.[15] While this remains possible, the unfusedsacralvertebrae and short face indicate that this specimen represents a juvenile animal. A second, larger specimen originally proposed to belong toAbrictosaurus clearly possesses tusks, which was used to support the idea that tusks are found only in adults, rather than being asecondary sexual characteristic of males. These tusks could have been used for combat ordisplay with members of the same species or with other species.[3] The absence of tusks in juvenileAbrictosaurus could also be another characteristic separating it from other heterodontosaurids as well, as tusks are known in juvenileHeterodontosaurus. Other proposed functions for the tusks include defense and use in an occasionally omnivorous diet.[53] However, this specimen was alternatively reassigned toLycorhinus by Sereno in 2012, which is already known to have possessed tusks and therefore their absence inAbrictosaurus may not have been a result of age.[1]
In 2005 a small complete fossilized heterodontosaurid skeleton more than 200 million years old was discovered inSouth Africa. In July 2016 it was scanned by a team of South African researchers using theEuropean Synchrotron Radiation Facility; the scan of the dentition revealed palate bones less than a millimeter thick.[54]
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