Paleontologists digging in the Sahara in central Niger have discovered a new species of gigantic plant-eating dinosaur, buried in a flash flood about 135 million years ago.

The University of Chicago's Paul Sereno said his team recovered 95 percent of the bones of what was once a 20-ton, 30-foot-tall animal. It is the most complete skeleton ever found of a sauropod--the family of four-legged, long-necked prehistoric herbivores that include such giants as Diplodocus and Apatosaurus, he said.

But Sereno said the new find, named Jobaria tiguidensis, is "remarkably primitive" for the Cretaceous period in which it lived, displaying spoon-shaped teeth and a relatively short neck and tail more common to dinosaurs that lived as much as 40 million years earlier.

Models of a Jobaria adult and a much smaller juvenile were put on display yesterday in the courtyard of the National Geographic Society, which helped fund the 1997 dig. Results of the excavation were published in today's edition of the journal Science.

Sereno also reported on the discovery of Nigersaurus taqueti, another plant-eater believed to be about 110 million years old. Nigersaurus's 600 teeth are pencil-shaped, typical of the Cretaceous. But with a maximum body length of about 50 feet, it is one of the smallest sauropods ever found.

Sereno said he first visited Niger on a 1990 dig, and found the Jobaria site during a 10-day exploratory visit to the Tiguidit Cliffs region near the town of Agadez.

"A Tuareg nomad took us to a place where he said we could see 'some big camel bones coming out of the ground,' " Sereno said in an interview. Jobaria is named after "jobar," a creature in Tuareg mythology.

Sereno and his team finished excavating the site in 1997, removing 20 tons of bone and rock in 1 1/2 months. Sereno said the team collected substantial portions of seven or eight Jobaria skeletons, including individuals of "three or four age groups."

Fossil remains in the area indicate that Jobaria dominated what seems to have been a "pleasant" environment of open forest with conifer-like trees and broad rivers with crocodiles and turtles, he said. By contrast, Sereno's 18-member team worked in 120-degree desert temperatures under a scorching Saharan sun.

Sereno was unable to determine how the dinosaurs died, but he said a rib from a juvenile Jobaria appears to have been scarred by tooth-marks, perhaps from a 27-foot carnivore called Afrovenator, the leading predator of the time.

Regardless of the cause of death, Sereno said, evidence recovered from the site strongly suggests that the animals' carcasses were washed together and buried during a flash flood. The adult Jobaria on display at National Geographic was lying on top of the juvenile.

Jobaria's size, about four times that of an African elephant, clearly puts it in "the top 25 percent" of all dinosaurs, Sereno said, but well short of the 36-ton Apatosaurus, the largest land animal that ever lived.

More striking, he added, Jobaria lacked many of the characteristics of other Cretaceous sauropods, especially the extremely long flexible necks, the whiplash tails and the narrow teeth capable of grinding many kinds of vegetation.

Instead, Jobaria was a much blockier creature, shorter in neck and tail, "though not ungraceful," Sereno said. He theorized that Jobaria could probably rise easily on two feet to nibble small tree branches, a task for which its spoon-shaped teeth are well suited.

Because these characteristics hark back to earlier Jurassic times, Sereno suggested that Jobaria probably diverged from the family tree of other broad-toothed sauropods as the continents drifted apart, a process that is believed to have ended about 150 million years ago.

What resulted, perhaps because it had no natural predators or because it had no need to adapt to changing environments, was a uniquely African animal that remained basically unchanged for about 40 million years. "It looks like a relic in its own time," Sereno said.