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Abrictosaurus
''Abrictosaurus'' (; "wakeful lizard") is a genus of heterodontosaurid dinosaur from the Early Jurassic Period of what is now southern Africa. It was a small bipedal herbivore or omnivore, approximately 1.2 meters (4 feet) long, and weighing less than 45 kilograms (100 pounds).
This dinosaur is known from the fossil remains of only two individuals, found in the Upper Elliot Formation of Qacha's Nek District in Lesotho and Cape Province in South Africa. The Upper Elliot is thought to date from the Hettangian and Sinemurian stages of the Early Jurassic Period, approximately 200 to 190 million years ago.〔Norman, D.B., Sues, H.-D., Witmer, L.M., & Coria, R.A. 2004. Basal Ornithopoda. In: Weishampel, D.B., Dodson, P., & Osmolska, H. (Eds.). ''The Dinosauria'' (2nd edition). Berkeley: University of California Press. Pp. 393-412.〕 This formation is thought to preserve sand dunes as well as seasonal floodplains, in a semiarid environment with sporadic rainfall. Other dinosaurs found in this formation include the theropod ''Megapnosaurus'', the sauropodomorph ''Massospondylus'', as well as other heterodontosaurids like ''Heterodontosaurus'' and ''Lycorhinus''. Remains of terrestrial crocodylomorphs, cynodonts and early mammals are also abundant.〔Weishampel, D.B. & Witmer, L.M. 1990. Heterodontosauridae. In: Weishampel, D.B., Dodson, P., and Osmolska, H. ''The Dinosauria'' (1st edition). Berkeley: University of California Press. Pp.486-497.〕
==Description==
Heterodontosaurids like ''Abrictosaurus'' were small, early ornithischians, named for their markedly heterodont dentition. They are best known for the large, canine-like tusks (often called ''caniniforms'') in both upper and lower jaws. There were no teeth in the front of the jaws, where a hard beak was used to crop vegetation. There were three premaxillary teeth, with the first two small and conical and the third enlarged to form the upper caniniform, counterpart to the even larger lower caniniform, which was the first dentary tooth. In the upper jaw, a large gap (or diastema) accommodated the lower caniniform tooth and separated the premaxillary teeth from the wider chewing teeth of the maxilla. Similar teeth lined the remainder of the lower jaw.〔
''Abrictosaurus'' is usually considered the most basal member of the family Heterodontosauridae.〔〔 ''Lycorhinus'' and ''Heterodontosaurus'' both had high-crowned cheek teeth, which overlapped each other in the jaw, forming a continuous chewing surface analogous to those of Cretaceous hadrosaurids. ''Abrictosaurus'' had more widely-separated cheek teeth, with lower crowns, more similar to other early ornithischians. It has been suggested that ''Abrictosaurus'' lacked tusks and that this is another primitive feature.〔Sereno, P.C. 1986. Phylogeny of the bird-hipped dinosaurs (Order Ornithischia). ''National Geographic Research'' 2: 234-256.〕 However, caniniforms were clearly present on one of the two specimens of ''Abrictosaurus''. The upper caniniform measured 10.5 millimeters (0.4 inches) high, while the lower reached 17 mm (0.67 in). These caniniforms were serrated only on the anterior surface, unlike those of ''Lycorhinus'' and ''Heterodontosaurus'', which were serrated on both anterior and posterior edges.〔Thulborn, R.A. 1970. The systematic position of the Triassic ornithischian dinosaur ''Lycorhinus angustidens''. ''Zoological Journal of the Linnean Society'' 49: 235-245.〕〔Hopson, J.A. 1975. On the generic separation of the ornithischian dinosaurs ''Lycorhinus'' and ''Heterodontosaurus'' from the Stormberg Series (Upper Triassic) of South Africa. ''South African Journal of Science'' 71: 302-305.〕 ''Abrictosaurus'' also had smaller, less powerful forelimbs than ''Heterodontosaurus'' and one fewer phalanx bone in both the fourth and fifth digits of the forelimb.〔Thulborn, R.A. 1974. A new heterodontosaurid dinosaur (Reptilia: Ornithischia) from the Upper Triassic Red Beds of Lesotho. ''Zoological Journal of the Linnean Society of London.'' 55: 151-175.〕
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