Fossils discovered in Tanzania in the 1930s have helped identify a "missing link" in dinosaur evolution that reveals their ancestors had long necks, walked on four legs and looked like crocodiles, AFP reports.

The 245-million-year-old fossils held in Britain's Natural History Museum were studied by palaeontologist Alan Charig in the 1950s.

But they were incomplete and it was only more fossils found in Tanzania in 2015 that helped build up the skeleton of "Teleocrator rhadinus".

"The finding forces a rethink of early dinosaur evolution," the journal Nature, where the research was published on Wednesday, said in a summary.

"The discovery of such an important new species is a once-in-a-lifetime experience," said Sterling Nesbitt, one of the authors and an assistant professor of geosciences at Virginia Tech in the US.

The international research project focuses on the evolution of a large group of reptiles called archosaurs into two branches: crocodiles and birds.

Dinosaurs evolved from the bird branch but the scientists found Teleocrators, which were already extinct by the time dinosaurs appeared, kept crocodile features and did not conform to a classic conception of early forms of dinosaur.