Agrosaurus (Seeley, 1891; meaning 'field lizard', referring to the place where it was supposedly discovered, (Greek agros meaning 'field' and sauros meaning '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 ship 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 Seeley identified it in that year. The block was prepared in the late 1980s. Following the preparation, Ralph Molnar (1991) noticed similarities to the prosauropod Massospondylus. Galton and Cluver (1976) see Agrosaurus as close to Anchisaurus. Vickers-Rich, Rich, McNamara and Milner (1999) equate 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.' Remains of the jaw of a sphenodont identical to Diphyodontosaurus avonis, a lizard-like reptile common to the Bristol Triassic beds has 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 3 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.