Fossil experts belong to two camps these days.

One group of paleontologists says the dinosaurs and other forms of life were snuffed out 65 million years ago by a huge asteriod or two.Another group says a gradual climate change triggered the dinosaurs' demise over millions of years and the leviathans remain the biggest success story in the history of life on the planet.

Who is right?

Maybe both, says Gary Upchurch.

Upchurch, a prehistoric plant researcher at the University of Colorado, said fossil evidence in a layer of clay shows that an asteroid impact apparently wiped out many plant species 65 million years ago.

"Something dramatic happened in conjunction with this boundary clay (and) there are marked changes compatible with a mass kill," Upchurch said. "But the demise of plant genera (classes) was more dependent on long-term climate change than any single event."

As many as two-thirds of the plant species in western North America may have been wiped out along with dinosaurs when one or two asteroids slammed into the planet, Upchurch said.

Impacts on land may have stirred up enough debris to block out the sun, while ocean impacts may have sufficiently altered water chemistry and temperatures to kill many forms of life, he said.

Upchurch reached his conclusions after examining whole leaf fossils and leaf fragments from western North American fossil beds.

The most widespread evidence is fern spores in a layer of clay that separates fossils from the Cretaceous and Tertiary geologic periods. At that time the western United States was a temperate region bordering on the tropics and ferns are the first plants to resprout after catastrophic events in those areas.

The element iridium has already been found in the clay. Iridium is rarely found in the earth's crust, but is a common component of meteors.