LOS ANGELES - More than 200 million years ago, toothy, crocodilelike creatures stalked a hot, dry megacontinent while squid-like mollusks with spiral shells drifted in the surrounding ocean.

Then, in an instant in geologic time, they vanished - making way for the age of the dinosaurs.

How 50 percent of terrestrial vertebrates and an even larger share of marine life died off in the late Triassic period has become clearer from new research published online Thursday in the journal Science.

The work lends greater validity to the theory that a massive volcanic event tore apart that continent and blanketed the atmosphere, turning the ocean acidic and snuffing out animals that could not adapt. That geologic event, which created the Atlantic Ocean, ushered in the biggest biological shift in the planet's history.

"It set the stage for the dinosaurs to take over, biologically," said Paul Olsen, a geologist at Columbia University's Lamont-Doherty Earth Observatory, who did much of the fieldwork on which the study is based.

Pushed by the nascent Atlantic, the ruptured pieces of Pangea drifted off and split further, carrying the evidence of ecological collapse to such distant locales as Morocco, Nova Scotia and New Jersey.

Matching the fossil record in sedimentary rock with the dense basalt formed by the volcanic eruptions proved difficult. It was hard to say whether the eruptions happened before the mass extinction.

Olsen and others, however, hacked rare zircon crystals from the basalt formations and measured traces of lead and uranium for radiometric dating tests. The results narrowed the margin of error in dating the lava to a mere 15,000 to 22,000 years - stunning precision for geochronology.

The crystals were not easy to find, or to process, although Olsen mined one from a rocky outcrop near his New Jersey home.

The mass extinction occurred 201.56 million years ago, around the age of one of the basalt rock formations dated by the team, according to the researchers.