A California paleontologist who has found the fossil bones of dinosaurs on Alaska's North Slope says his discovery poses a challenge to the theory that all dinosaurs were wiped out by an asteroid impact 65 million years ago. In recent years, many scientists have been impressed with evidence that a giant asteroid hit the Earth back then, blasting up enough dust to darken the sky for weeks or months and inducing an abnormal winter that could have caused the mass extinction that apparently occurred.

William Clemens of the University of California, Berkeley, says the land where the dinosaurs lived was at least as far north in those days as now and that, therefore, the dinosaurs would have been adapted to survive the same months of winter darkness that affect the region today. Thus, he says, the darkness could not have killed them.

Clemens says the freeze that resulted also would not have affected the dinosaurs because the warm ocean would have moderated it in the coastal region where the dinosaurs lived. The whole Earth then was warm, and Alaska had a subtropical climate.

Thus, Clemens concludes, something other than an asteroid impact must have caused the Alaskan dinosaurs to become extinct.

Most adherents of the asteroid theory remain unpersuaded by Clemens's case because it rests on many assumptions, especially that the asteroid-triggered freeze would not have dropped north Alaskan temperatures below freezing.

Clemens, who has criticized the asteroid theory before, holds that there never was a mass extinction. Instead, he says, the extinctions occurred one at a time or in small groups over widely spaced intervals.