|Cast of the holotype specimen SAM-PK-K1332 of Heterodontosaurus tucki, Cape Town|
They 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 Upper Triassic and may have persisted into the Lower Cretaceous.
Jaws and teethEdit
The mandible (lower jaw) was tipped by the predentary, a bone unique to ornithischia. 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.
Heterodontosaurids are called that because they have two different kinds of teeth. There were three premaxillary teeth. In the early Jurassic Abrictosaurus, Heterodontosaurus, and Lycorhinus, the first two premaxillary teeth were small and conical, while the much larger third tooth resembled the canines of carnivoran mammals and is often called the caniniform or 'tusk'.
Several early studies suggested that heterodontosaurs were primitive ornithischians. The main hypothesis in recent years has put heterodontosaurids in as basal ornithopods. However, others have suggested that heterodontosaurs share a common ancestor with Marginocephalia (ceratopsians and pachycephalosaurs). This has support from some recent studies. A clade containing heterodontosaurs and marginocephalians has been named Heterodontosauriformes. Heterodontosaurs may even be basal to both ornithopods and marginocephalians. In 2007, a cladistic analysis suggested that heterodontosaurs are basal to all known ornithischians except Pisanosaurus, a result that echoes some of the very earliest work on the family.
- Sereno, Paul C. (1986). "Phylogeny of the bird-hipped dinosaurs". National Geographic Research. 2 (2): 234–256.
- Zhao Xijin (1983). "Phylogeny and evolutionary stages of Dinosauria". Acta Paleontologica Polonica. 28 (1–2): 295–306.
- Cooper, Michael A. (1985). "A revision of the ornithischian dinosaur Kangnasaurus coetzeei Haughton, with a classification of the Ornithischia". Annals of the South African Museum. 95: 281–317.
- Xijin, Zhao; Zhengwu, Cheng; Xing, Xu (1999). "The earliest ceratopsian from the Tuchengzi Formation of Liaoning, China". Journal of Vertebrate Paleontology. 19 (4): 681–691. doi:10.1080/02724634.1999.10011181.
- You Hailu; Xu Xing; Wang Xiaolin (2003). "A new genus of Psittacosauridae (Dinosauria: Ornithopoda) and the origin and early evolution of marginocephalian dinosaurs". Acta Geologica Sinica (English Edition). 77 (1): 15–20.
- Xu, Xing; Forster, Catherine A.; Clark, James M.; Mo, Jinyou (2006). "A basal ceratopsian with transitional features from the Late Jurassic of northwestern China". Proceedings of the Royal Society B: Biological Sciences. 273 (1598): 2135–2140. doi:10.1098/rspb.2006.3566. PMC 1635516. PMID 16901832.
- Maryanska, Teresa; Osmólska, Halszka (1985). "On ornithischian phylogeny". Acta Paleontologica Polonica. 30: 137–150.
- Butler, Richard J. (2005). "The 'fabrosaurid' ornithischian dinosaurs of the Upper Elliot Formation (Lower Jurassic) of South Africa and Lesotho". Zoological Journal of the Linnean Society. 145 (2): 175–218. doi:10.1111/j.1096-3642.2005.00182.x.
- Butler, Richard J.; Smith, Roger M.H; Norman, David B. (2007). "A primitive ornithischian dinosaur from the Late Triassic of South Africa, and the early evolution and diversification of Ornithischia". Proceedings of the Royal Society B: Biological Sciences. 274 (published online): 2041–6. doi:10.1098/rspb.2007.0367. PMC 2275175. PMID 17567562.
- Butler, Richard J.; Upchurch, Paul; Norman, David B. (2008). "The phylogeny of the ornithischian dinosaurs". Journal of Systematic Palaeontology. 6 (1): 1–40. doi:10.1017/S1477201907002271. S2CID 86728076.