Heterodontosauridae

of Tianyulong, showing filamentous integument known from the taxon 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 most 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. Heterodontosaurus measured just over 1 meter (3.3 ft) in length, while the fragmentary remains of Lycorhinus may indicate a larger individual. 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 were endothermic. Skull and teeth 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 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 dentary bone. In the Early Cretaceous Echinodon, there may have been two upper caniniforms, which were on the maxilla rather than the premaxilla, and Fruitadens from the Late Jurassic may have had two lower caniniforms on each dentary. Like the characteristic tusks, the cheek teeth of derived 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 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 derived forms 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. Skeleton 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 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 tibia was 30% longer than the femur, which is generally considered an adaptation for speed. The tibia and fibula of 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. ==Classification==

jawbone of Geranosaurus South African paleontologist Robert Broom created the name Geranosaurus in 1911 for dinosaur jaw bones missing all of the teeth and some partial associated limb bones. In 1924, Lycorhinus was named, and classified as a cynodont, by Sidney Haughton. Heterodontosaurus was named in 1962 and it, Lycorhinus and Geranosaurus were recognized as closely related ornithischian dinosaurs. Alfred Romer named Heterodontosauridae in 1966 as a family of ornithischian dinosaurs including Heterodontosaurus and Lycorhinus. Kuhn independently proposed Heterodontosauridae in the same year and is sometimes cited as its principal author. It was defined as a clade in 1998 by Paul Sereno 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. Heterodontosauridae was given a formal definition in the PhyloCode by Daniel Madzia and colleagues in 2021 as "the largest clade containing Heterodontosaurus tucki, but not Iguanodon bernissartensis, Pachycephalosaurus wyomingensis, Stegosaurus stenops, and Triceratops horridus". but more recent discoveries have shown that it belongs to Lycorhinus instead, making Lanasaurus a junior synonym of that genus. but it has since been shown that the remains were a chimera of prosauropod and mesoeucrocodylian remains. José Bonaparte also classified the South American Pisanosaurus as a heterodontosaurid at one time, but this animal is now known to be a more basal ornithischian. '' The membership of Heterodontosauridae is well-established in comparison to its uncertain phylogenetic position. Several early studies suggested that heterodontosaurids were very primitive ornithischians. However, others have suggested that heterodontosaurids instead share a common ancestor with Marginocephalia (ceratopsians and pachycephalosaurs), a hypothesis that has found support in some early 21st century studies. The clade containing heterodontosaurids and marginocephalians has been named Heterodontosauriformes. Heterodontosaurids have also been seen as basal to both ornithopods and marginocephalians. In 2007, a cladistic analysis suggested that heterodontosaurids are basal to all known ornithischians except Pisanosaurus, a result that echoes some of the very earliest work on the family. However, a study by Bonaparte found the Pisanosauridae to be synonymous with the Heterodontosauridae and not a separate family in its own right, thereby including Pisanosaurus as a heterodontosaur. Butler et al. (2010) found the Heterodontosauridae to be the most basal known significant ornithischian radiation. The cladogram below shows the interrelationships within Heterodontosauridae, and follows the analysis by Sereno, 2012: }} A 2020 reworking of Cerapoda 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 the dentition, and also to account for the fact that pachycephalosaur fossils are completely unknown from the Jurassic period. Modern understanding of ornithischian phylogeny implies that Jurassic pachycephalosaurs must exist, because numerous Jurassic ceratopsians 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 found Chilesaurus to be a basal ornithischian. The phylogenetic analysis was conducted with Chilesaurus coded as an ornithischian, which also had implications for the phylogeny of ornithopods. Similar results were also found by Dieudonné et al. (2026). The cladogram below is an abridged version of Dieudonne and colleagues' findings: }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} ==Distribution==

distribution of heterodontosaurids in time 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 Pangaea was still largely intact, allowing the family to achieve a near-worldwide distribution. Irmis et 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, while Sereno (2012) only stated that this material may represent an ornithischian or even specifically a heterodontosaurid. 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. The most diverse heterodontosaurid fauna comes from the Early Jurassic of southern Africa, where fossils of Heterodontosaurus, Abrictosaurus, Lycorhinus, and the dubious Geranosaurus are found. and Mexico, respectively. In addition, beginning in the 1970s, a great deal of fossil material was discovered from the Late Jurassic Morrison Formation near Fruita, Colorado in the United States. as well as the equivalently aged Itat Formation of Western Siberia. Heterodontosaurid teeth lacking a cingulum have also been described from Late Jurassic and Early Cretaceous formations in Spain and Portugal. The remains of Echinodon were redescribed in 2002, showing that it may represent a late-surviving heterodontosaurid from the Berriasian stage of the Early Cretaceous in southern England. ==Paleobiology==

'' Most heterodontosaurid fossils are found in geologic formations that represent arid to semi-arid environments, including the Upper Elliot Formation of South Africa and the Purbeck Beds of southern England. It was thought that heterodontosaurids actually did replace their teeth continually, though more slowly than in other reptiles, but CT scanning of skulls from juvenile and mature Heterodontosaurus shows no replacement teeth. There is currently no evidence that supports the hypothesis of aestivation in heterodontosaurids, ==References==