Discovered in southern Brazil, in the geopark of Paleorrota, in 1998, and announced in a press conference on Thursday, December 3, 2004, it is one of the oldest dinosaurs known.

In 2004, Luciano A. Leal, Sergio A. K. Azevodo, Alexander W. A. Kellner, and Átila A. S. da Rosa described Unaysaurus tolentinoi as a new genus and species on the basis of the holotype specimen UFSM 11069.

Under this assignment, the closest relative of Unaysaurus was, counter intuitively, not from South America, but rather Plateosaurus, which lived about 210 million years ago in Germany.

[3] Unaysaurus lived between about 225 to 200 million years ago, in the Carnian or Norian age of the late Triassic period.

Multiple dinosauriforms are represented in the rock of the formation, including the silesaur Sacisaurus agudoensis, and the coeval sauropodomorph Guaibasaurus candelariensis,[5] all of which are not found anywhere else.

Species preserved are the procolophonid Soturnia caliodon, the lepidosaur Cargninia enigmatica, the sphenodontid Clevosaurus brasiliensis, and some small therapsids coexisting with Faxinalipterus minima, a putative pterosaur.