During the so-called Permian-Triassic (P-T) extinction, 90% of all marine species, including the last of the trilobites, abruptly disappeared. On land, pervasive extinctions opened the way for the rise of the dinosaurs. Many paleontologists presume it all had a single, abrupt cause, such as a gargantuan volcanic eruption or a cosmic impact. But no one had been able to implicate such a catastrophe by placing it at the geologic moment of extinction.

Geochemist Luann Becker of the University of Washington in Seattle believes a cosmic impact may leave traces in the form of ball-shaped, pure-carbon molecules called fullerenes, which Becker has found in two meteorites. Now Becker and her colleagues have discovered fullerenes in P-T boundary rocks at sites in South China and southwest Japan, as they report in the 23 February issue of Science. Similar rocks just above or below the boundary didn't have the fullerenes.

The clincher, Becker says, lies inside. Because of their hollow structure, fullerenes can trap gas atoms like birds in a cage. Inside the P-T fullerenes, the researchers detected 50 times more helium-3 than above or below the boundary. The ratio of helium-3 to helium-4 entrapped there was typical of that found in meteorites--not in the atmosphere or rock. The same was true of the argon-40 to argon-36 ratio. The findings are "the best case for an extraterrestrial event coincident with the P-T extinction," Becker says.

Fullerene experts aren't sure that the molecules can survive billions of years. "The work of Luann Becker and colleagues has been a bit controversial," notes microscopist Peter Harris of the University of Reading, United Kingdom. Among noble gas workers, the reception has been warmer. Geochemist Kenneth Farley of the California Institute of Technology in Pasadena is astounded by the argon ratio: "I can't imagine how you could have any other interpretation" than an impacting meteorite that carried in the noble gases.

--RICHARD A. KERR