About three dozen minuscule shards of rock unearthed in Antarctica may be the fragments of a meteor that killed most life on Earth 250 million years ago, scientists are reporting today.

The shards bolster theories that meteors caused several mass extinctions. Scientists generally agree that the most recent major mass extinction, 65 million years ago, which killed off the dinosaurs, was caused when a meteor struck near the Yucatán Peninsula in Mexico.

The extinction 250 million years ago, in a period known as the Permian-Triassic boundary, was the largest of all. About 90 percent of species disappeared.

At present, the primary suspected cause for the Permian-Triassic extinction is giant eruptions in Siberia, which might have induced catastrophic ecological changes.

Writing in today's issue of the journal Science, the researchers report that they found the meteorite fragments in rocks in Antarctica that date from the Permian-Triassic boundary. The mineral composition of the fragments, each less than one-fiftieth of an inch wide, correspond to that of certain meteorites and is like nothing found naturally on Earth, they reported...

...Others are not yet convinced. Dr. Eldridge Moores, an emeritus professor of geology at the University of California at Davis, described the meteorite fragments as "the most interesting evidence for a meteorite event at this boundary that I've seen so far." But, he added, while the evidence for the dinosaur-killing meteor 65 million years ago is a convincing 10 on a 1-to-10 scale, the evidence for a killer meteor at the Permian-Triassic boundary is not nearly as solid.

"I think it's now up to 3 or 4," Dr. Moores said. "It's not 9 or 10."

Dr. Douglas H. Erwin of the National Museum of Natural History in Washington said, "It's suggestive, but it's hardly compelling."

Each piece of evidence offered so far has not by itself been compelling...