Dinosaurs Survived One Mass Extinction

May 10 -- The dinosaurs were wiped outby an asteroid that smacked the Earth 65 million years ago, butthey survived another cataclysmic event — perhaps anotherasteroid impact — that snuffed out 80 percent of all speciesabout 200 million years ago, scientists said today.

By studying the fate of a type of marine plankton,single-celled organisms called Radiolaria, researchers foundthat the mass extinction was a sudden event, not the prolongeddie-off that experts previously had thought. The extinctionoccurred at the boundary between the Triassic and Jurassicperiods during the Mesozoic era.

The event provided the death knell for most species andhelped crown the dinosaurs, which arose earlier in theTriassic, as the rulers of the Earth, said Peter Ward, aUniversity of Washington paleontologist who led the study.

Ward said this calamity had tremendous similarities to twoof the other five mass extinctions that have ravaged Earth overthe past 500 million years. Like those, Ward said it appearsthis mass extinction was caused by a giant rock from space.

"We know now that asteroid impact can cause rapidextinction," Ward said in an interview. "It may not be anasteroid. But if it isn't an asteroid, it acts like anasteroid, put it that way."

Wiped Out in a Second?

Most scientists believe an asteroid strike caused the massextinction at the end of the Cretaceous period that killed thedinosaurs and ushered in the age of mammals. In February,scientists presented evidence that an asteroid or comet impactalso caused the even bigger extinction at the boundary betweenthe Permian and Triassic periods 250 million years ago.

Ward's team gathered evidence about the extinction 199.6million years ago at two remote sites in the Queen CharlotteIslands off Canada's British Columbia coast, examining fossilsamples indicating a collapse of the plankton population.

The researchers found an abrupt drop in the rate at whichinorganic carbon was turned into organic carbon by life formsthrough processes such as photosynthesis.