The fossils - which include two skulls and other bones - date back to the high Triassic period, between 235 and 240 million years ago, when the dinosaurs were first developing from older reptiles called Thecodonts.
Palaeontologists in Brazil say they may have found the oldest known dinosaur fossil, in the south of the country.
The fossils of the creature could provide important clues as to how the dinosaurs evolved.
The newly-discovered creature had a pointed head of about 30 centimetres (12 inches) and was more than two metres (6.5 feet) long. It had the sharp teeth of a carnivore.
The fossils - which include two skulls and other bones - date back to the high Triassic period, between 235 and 240 million years ago, when the dinosaurs were first developing from older reptiles called Thecodonts.
It is a period of evolution about which there is little evidence and which has generated sharp debate among palaeontologists.
Evolutionary link
The Thecodonts, which moved on all fours, were the direct ancestors of crocodiles, but they were replaced by the faster and more agile dinosaurs.
The new find appears to have some characteristics of the Thecodonts and some of the dinosaur, and so may offer vital clues as to the evolutionary chain.
It was found in Rio Grande de Sul province, in southern Brazil, by a team led by palaeontologist Jorge Ferigolo.
He says he expects to publish his first findings of it within a year, although the studies will continue for much longer.
It is no surprise that the find should have been made in this part of South America.
Brazil and northern Argentina have long provided some the best dinosaur finds.
The area is a treasure trove for dinosaur fossils - last year palaeontologists found another of the oldest dinosaurs yet discovered, a direct ancestor of the Tyrannosaurus rex.