The dinosaur would have occasionally walked on all four legs
Researchers have discovered a fossil skeleton that appears to link the earliest dinosaurs with the large plant-eating sauropods.
This could help to bridge an evolutionary gap between the two-legged common ancestors of dinosaurs and the four-legged giants, such as diplodocus.
The remarkably complete skeleton shows that the creature was bipedal but occasionally walked on all four legs.
The team reports its discovery in the Royal Society journal Proceedings B.
"What we have is a big, short-footed, barrel-chested, long-necked, small-headed dinosaur," explained Adam Yates, the scientist from the University of Witwatersrand in Johannesburg who led the research.
"The earliest ancestral dinosaur - the great grand-daddy of all dinosaurs - walked on two legs. This [one] is intermediate between those bipedal forms and the true gigantic sauropods."
The dinosaur had a wide gape "to stuff more food in"
The skeleton was discovered at a site in the Senekal district of South Africa.
Dr Yates explained that features of its feet and jaw, as well as its size, gave away its significance.
The dinosaur, Aardonyx celestae was a heavy, slow-moving animal.
"It had a lot of features we see on sauropods," explained Dr Yates. "Short, broad feet and a big, broad gut, so it was clearly a plant-eater that was bulk-feeding.
"And the anatomy of the jaw shows it had a wide gape - to stuff more food in."
It also had, he said, "sauropod-like front feet".
"Its toe bones were very robust and solid, so its weight was being born on the inside of the foot. It was still bipedal, but it may have been going down on to all fours to browse."
The dinosaur dates from the early Jurassic period - about 200 million years ago.
"Although structurally it's intermediate, it lived too late to be an actual ancestor, because true sauropods already existed [then].
"So, at the time, it was a living fossil - the transition must have happened much earlier."
Dr Yates stressed that the site where the fossil was discovered provided an abundance of valuable knowledge about dinosaur evolution.
"If you want to study how the dinosaurs became giants," he said. "You have to come to South Africa."
Dr Paul Barrett - a palaeontologist from the Natural History Museum in London said that the discovery of Aardonyx helped "fill a marked gap in our knowledge of sauropod evolution".
"[It shows] how a primarily two-legged animal could start to acquire the specific features necessary for a life spent on all fours.
"Evolution of this quadrapedal gait was key in allowing the late sauropods to adopt their enormous body sizes."