Johannesburg - South African palaeontologists on Wednesday announced the discovery of a new species of "transition" dinosaur that straddles the divide between the four-legged giant plant-eating sauropods and their bipedal carnivorous predecessors.

The Aardonyx Celestae, as the new dinosaur has been named, was discovered on a farm in central Free State province in the Karoo Basin, an area rich in fossils.

"What we have here in Aardonyx is an intermediary dinosaur. It's not entirely a prosauropod and it's not a sauropod," palaeontologist Adam Yates said, unveiling the fossil remains at the University of the Witwatersrand in Johannesburg.

The dinosaur roamed the area between 183 million and 200 million years ago, measured between 7 and 9 metres long and was "probably the weight of a horse," he said. The specimen was a juvenile, which was aged between 7 and 10 years.

Aardonyx dates to the early Jurassic period and has many features of the sauropod - the huge, lumbering dinosaurs with the small heads, long necks and elephantine legs popularly known as brontosaurs that dominated the Earth during that period.

A study of the bones found showed it had a small head, big barrel chest and was slow-moving and plant-eating, Yates said.

It also had the wide, gaping mouth of a browser and foot bones that become thicker towards the inside of the foot - characteristics of a sauropod, which supported its weight on the inside of the foot.

But its short forearms suggested it was a biped.

Its narrow pointed jaw was also more in keeping with the smaller meat-eating prosauropods that predated the sauropods and dominated in the Triassic period,

"Aardonyx gives us a glimpse into what the steps towards becoming a sauropod involved," he added.

While dating to the Jurassic period, Aardonyx was probably already by then "a living fossil", Yates said.

The discovery is the latest in a string of finds of sauropods and prosauropods in the Karoo Basin.

"We have here in South Africa the cradle of Sauropod-kind," Yates, who is one of only six palaeontologists in South Africa, said triumphantly, referring to the Karoo Basin.

In 2003, a 215-million-year-old sauropod from the Late Triassic period named the Antetonitrus was also discovered in the Karoo Basin.