The fossils, discovered in a rock formation in Niger, are of a primitive 20-ton animal that was more than 70 feet long.
A team led by Paul Sereno of the University of Chicago excavated tons of bone and rock and then spent two years cleaning and assembling the specimen. A report on the find appears Friday in the journal Science and an assemblage of the fossils is on display at the National Geographic Society headquarters.
The previously unknown dinosaur has been named Jobaria tiguidensis. Researchers believe it lived about 135 million years ago at a time when the Sahara Desert was a lush forest with broad rivers.
Jobaria resembles the Apatosaurus, a sauropod that roamed North America around the same time. But Jobaria has a primitive, simple bone structure, suggesting that the animal represents a less evolved form of sauropod.
"It is kind of like a relic," said David Varricchio of Montana State University. "It looks like it should have existed 40 million years before it did."
Varricchio is one of 11 co-authors of the dinosaur study.
Jeff Wilson of the University of Michigan, another co-author, said Jobaria was "a real survivor" that seemed to have evolved at a slower pace.
"Some dinosaurs change a lot in a short amount of time, whereas others, like Jobaria, change very little over millions of years," Wilson said in a statement.
The fact that Jobaria's feet are set close together under its body suggests that it could move gracefully, considering its size, Sereno said in a statement.
"Its proportions were elephant-like and its bones could have supported its body mass when rearing during feeding or in courtship contests," said Sereno.
The rock formation in Niger held fossils of several adults and juveniles. The researchers said this suggests that the Jobaria lived in herds of mixed ages. The whole group was apparently buried together by a flash flood.
One of the juvenile dinos carried tooth marks on a rib, suggesting that it had been attacked by a meat-eater. Earlier, Sereno's team excavated the fossils of a 27-foot-long predator called Afrovenator from the same general area.