Scientists excavating a remote valley on the African island of Madagascar have unearthed jawbones from what may be the two oldest dinosaurs ever discovered.

Teeth from the new creatures indicate that they were both long-necked prosauropods, plant-eating ancestors of later--and much larger--herbivores such as the 36-ton Apatosaurus, the largest animal ever.

Evidence from surrounding fossils suggests that the new jawbones must be around 230 million years old, which would make them the oldest dinosaur remains ever found, said paleontologist John J. Flynn, who is leading the four-year excavation in southwestern Madagascar.

"They were both bipedal and quadripedal, somewhere between four and eight feet long," said Flynn, a curator at Chicago's Field Museum. "The kangaroo is a good visual image, because while they could use four legs to run, they could also forage with their front arms."

The excavation report, published by Flynn and five others in today's issue of the journal Science, also describes eight species of reptiles and mammal-like reptiles, some of which appear to be cousins of similar, already discovered species and which were key in fixing the age of the dinosaur remains. "There's still a lot of work to be done," Flynn said. "We have many skeletal parts that are jumbled together. We have to clean them up and begin to piece together each of the different species."

Even now, however, the site at Madagascar's Morondava Basin is attracting attention because of its wealth of fossil material from the little-known middle to late Triassic period, which ended about 200 million years ago.

At the beginning of the Triassic, the Earth's terrestrial fauna included a variety of reptiles, amphibians and other vertebrates, Flynn said. By the end, dinosaurs were firmly established and ready to dominate the ensuing Jurassic period until their mysterious extinction 135 million years later. Scientists regard the in-between Triassic years as critical because of the emergence of proto-mammals from earlier animals almost indistinguishable from reptiles, and because of the beginnings of specialization among dinosaurs. But the record has been sparse, Flynn continued, in part because of a shortage of known fossil-bearing deposits of the right age and in part because "the logistics are difficult."

Madagascar, a large Indian Ocean island, was once part of southern Africa but separated from the mainland when the continents were pulling apart far back in geologic time. This process created rift valleys--large, slumping, ditches that were formed when the sagging land mass did not fall below sea level. The resulting basins were filled with rivers and lakes--and highland sediments that hardened around the carcasses of dead plants and animals.

"This is the best terrain we can find for dinosaurs," Flynn said, explaining that since Madagascar broke away earlier than most parts of the supercontinent known to geologists as Pangea, the rift valleys collected rich Triassic deposits. And because of the island's early isolation from the African mainland, 85 percent of the animal species that live in Madagascar today live nowhere else. "There was virtually no fossil record in Madagascar from 10,000 years ago until 250 million years ago," Flynn said. "I thought it would be a good place to look."

Funded by the National Geographic Society and working jointly with archaeologists from Madagascar's Antananarivo University, Flynn's team has opened sites in three different areas dating back to the Triassic and Jurassic periods. Flynn said the team noticed that several of the reptiles and mammal-like reptiles found with the jawbones were thought to have died around the time dinosaurs appeared, suggesting the dinosaur remains were uncommonly old.

Also, he continued, one of the reptiles and one of the mammal-reptiles appeared to be more primitive versions of animals that had been found in Argentina with 227 million-year-old dinosaur remains.

The Argentine find--the oldest remains until now--was dated with radioisotopes, but anthropologists often can't use such techniques because the sedimentary rocks encasing many fossils had come from much older stone. Flynn said the Madagascar excavation has not yet reached datable deposits.

Finally, he added, the team expected to encounter two kinds of armored reptiles common in most Triassic sites, "but we don't have them." The team concluded that the Madagascar site is older than 227 million years--perhaps as much as five million years older.

CAPTION: OLDER ANCESTORS?

Fossilized teeth indicate the dinosaurs were long-necked prosauropods, between four and six feet long, and were the plant-eating ancestors of larger herbivores, such as the Apatosaurus.

SOURCES: Field Museum, Random House Encyclopedia

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CAPTION: Scientists in search of fossils excavate a site in Madagascar. The U.S.-led team has unearthed what may be the oldest dinosaur bones ever discovered.