Scientists say they have found the first complete skeleton of a dinosaur that is an ancestor to the sauropod, the largest creature to walk the planet.
The 30ft-long skeleton, complete with skull, may help reveal the story of how the 120ft-long giant-necked sauropods evolved and became solely plant eaters.
The bones were found in southern China, Texas Tech paleontologist Sankar Chatterjee said.
He plans to present preliminary findings of the discovery at the Geological Society of America meeting in Denver, Colorado, on Sunday.
"It's like a manuscript. Many pages are missing. We are still trying to piece it together," Professor Chatterjee said. "This is one of those missing links. Now we'll know better how these dinosaurs evolved."
The skeleton, estimated at more than 200 million years old, was found by one of Prof Chatterjee's colleagues in 2005 in flood plains around Lufeng in Yunnan province, which has been rich with dinosaur finds, he said.
It appears to be that of a new species, which has tentatively been named Yizhousaurus sunae to honour the local population and a Chinese paleontologist, Prof Chatterjee said.
At least six other species of sauropod relatives have been found in the same deposit where this skeleton was found, said Randall Irmis, curator of paleontolgy at the Utah Musuem of Natural History and an assistant professor at the University of Utah.
He said he had not seen Prof Chatterjee's report but saw photos of the skull. "It looks strikingly similar to an animal in the same deposits, the Jingshanosaurus," he said.
The skeleton Prof Chatterjee is studying shows the animal stood on four feet. It also has the beginnings of a long neck. The broad, high, domed skull has eye sockets on the sides and a short snout with a wide, U-shaped jaw of a broad. Serrated, spoon-shaped teeth suggest it ate plants, like sauropods.
The bones of all sauropod ancestors that scientists have found in recent years are filling in clues about how such giant beasts came to be and when they started eating only plants.
"Everyone loves sauropods because they're the largest animals to have ever walked the earth," Dr Irmis said. "Even the public wants to know, how did this great body size evolve? Specimens like this help us answer those questions."