New discoveries of dinosaur fossils on Alaska's North Slope support the claim, first advanced in 1985, that some dinosaurs could have survived the effects of the hypothetical asteroid impact that a recent theory suggests triggered a mass extinction 65 million years ago.

It is well established that a mass extinction took place at that time, wiping out many species all over the world. No dinosaur fossils from after that time are known, and paleontologists have generally concluded that the dinosaurs all perished in the same mysterious event.

The asteroid theory holds that a huge object crashed into Earth and blasted enough dust into the sky to block sunlight for weeks or months. The darkness and resultant cold would, in theory, have wiped out plant life, depriving animals of food.

The bones, representing hundreds of individual dinosaurs, were found in strata laid down before the mass extinction. Their challenge to the asteroid theory lies in the fact that the animals lived normally at a northward latitude that sustained months of darkness and episodes of freezing weather every winter.

Thus, according to Elisabeth Brouwers of the U.S. Geological Survey in Denver and William Clemens of the University of California at Berkeley, the Alaskan dinosaurs were not likely to have been fazed by the effects of an asteroid impact.

Alaska in those days was, if anything, farther north than it is now (continental drift moves the land masses around continually), but, because Earth was not in an Ice Age, the climate was mild compared with today. Fossil pollens found with the bones are those of herbaceous flowering plants and deciduous conifers such as redwoods.

The plants suggest that, despite the darkness and occasional frost, "weeks or months of subfreezing conditions are doubtful."

Brouwers and Clemens suggest in their report in last week's Science that the dinosaurs did not migrate south for the winter because some of the fossils were of very young dinosaurs that would have been unlikely to migrate long distances.

They conclude that the dinosaurs, which were mostly plant-eating, duck-billed hadrosaurs, must have had natural abilities to endure the dark winters and, therefore, were able to ride out the effects of an asteroid impact. Aquatic vegetation and branches of dormant shrubs could have sustained the great reptiles, the two say.

Brouwers and Clemens plan to search younger geologic deposits on the North Slope in hopes of finding the remains of dinosaurs that survived whatever it was that happened 65 million years ago.