Agrosaurus (//; Greek agros meaning 'field' and sauros meaning 'lizard', "field lizard") is the name given to the remains of what was originally believed to be a Triassic prosauropod from Australia. Agrosaurus would thus be the oldest dinosaur from that country. However, this appears to have been an error, and the material actually appears to come from Thecodontosaurus or a Thecodontosaurus-like animal from Bristol, England. The type species is Agrosaurus macgillivrayi.
Members of an expedition from the British sloop HMS Fly supposedly collected a tibia, a claw and some other fragments in 1844 from Cape York, Queensland. The original block was purchased by the British Museum of Natural History in 1879, but the remains were not studied until 1891. Harry Govier Seeley in that year named it Agrosaurus macgillivrayi. The block was prepared in the late 1980s. Following the preparation, Ralph Molnar (1991) noticed similarities to the prosauropod Massospondylus. Galton and Cluver (1976) saw Agrosaurus as close to Anchisaurus. Vickers-Rich, Rich, McNamara and Milner (1999) equated Agrosaurus and Thecodontosaurus antiquus, claiming that the British Museum remains were mislabelled. The difficulty in correctly identifying the source of the fossil lies in the fact that the log of the Fly does not record it. The matrix in which the prosauropod bones were preserved was tested with rocks of similar age in Cape York and Durdham Downs, the latter being beds where Thecodontosaurus remains have been found in the Bristol area of England. The English beds compared most favourably. In fact, as early as 1906 Friedrich von Huene had described the rock matrix as 'extremely reminiscent of the bone breccia at Durdham Downs near Bristol' and had renamed the species Thecodontosaurus macgillivrayi. Remains of the jaw of a sphenodont identical to Diphyodontosaurus avonis, a lizard-like reptile common to the Bristol Triassic beds have been extracted. This reinterpretation of Agrosaurus as a misidentified British specimen has been accepted in later works.
From the scant remains the living animal would appear to have been about three metres long (10 ft), with a typically prosauropodan appearance: bulky body, long neck, small head and clawed feet. Like other prosauropods, it was probably equally comfortable on all fours as well as on its elongated hind legs. It was herbivorous or may have been an omnivore.
The name Agrosaurus is now generally considered to be a nomen dubium or a junior synonym of Thecodontosaurus. If Agrosaurus is not from Australia, which seems most probable, Rhoetosaurus and Ozraptor, both from the Bajocian (Middle Jurassic) would be the oldest known Australian dinosaurs. Fortunately they are well documented.
- H. G. Seeley. 1891. On Agrosaurus macgillivrayi (Seeley), a saurischian reptile from the N.E. coast of Australia. Quarterly Journal of the Geological Society of London 47:164-165
- Weishampel, David B.; Barrett, Paul M.; Coria, Rodolfo A.; Le Loueff, Jean; Xu Xing; Zhao Xijin; Sahni, Ashok; Gomani, Elizabeth M.P.; Noto, Christopher N. (2004). "Dinosaur distribution". In Weishampel, David B.; Dodson, Peter; Osmólska, Halszka. The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 517–606. ISBN 0-520-24209-2.
- Galton, Peter (2007). "Notes on the remains of archosaurian reptiles, mostly basal sauropodomorph dinosaurs, from the 1834 fissure fill (Rhaetian, Upper Triassic) at Clifton in Bristol, southwest England". Revue de Paléobiologie. 26 (2): 505–591.
- John A. Long, Dinosaurs of Australia and New Zealand, UNSW Press 1998
- Vickers-Rich, P., T.H. Rich, G.C. McNamara and A. Milner 1999 Agrosaurus: Australia's Oldest Dinosaur? Records of the Western Australian Museum Supplement No.57: 191-200