RESEARCHERS at the University of Washington here say they have turned up new evidence that challenges the leading notions of how the dinosaurs became extinct. The death of clams about 1 million to 2 million years before the large reptiles disappeared suggests major changes in ocean circulation and chemistry that point to a gradual, worldwide climatic change. For a decade or more scientists have been arguing whether or not sudden disasters - a meteor colliding with Earth, volcanic eruptions, or others - caused the extinctions. In those scenarios a thick cloud formed, blocking sunlight and cooling the planet.
But the new evidence suggests a change in sea-water chemistry, affecting both water and land plants and making the dinosaurs and thousands of other species ripe for a fall. In this case, volcanoes or a meteorite collision would have merely added to the trauma of species already compromised.
Reporting the new information at a recent meeting of the Geographical Society of America in Dallas, University of Washington geological sciences Prof. Peter Ward said: ``If this oceanic event had not occurred, the dinosaur extinctions might not have either.''
Beginning in 1980, Dr. Ward and graduate student Ken MacLeod collected scores of extinct clam fossils known as inoceramids from the stratified cliffs along the Bay of Biscay in Spain and France. Correlating the extinction of the creatures to a fast rise in the levels of strontium (a metallic chemical element) in sea water, the scientists opened the door to new explanations: (1) a drastic climate change denuded the land of plants, causing extensive erosion which carried strontium to the sea; or (2) the sea level dropped, exposing more of the continent to erosion.
The new theory does not rule out the possibility that a meteorite or asteroid collided with Earth. Geological evidence of such an impact has been growing.
``But I do not believe that a single, large impact caused all of the extinctions,'' said Ward.
A. V. Murali, a scientist at the Lunar and Planetary Institute in Houston, says: ``Ward's and MacLeod's research is significant because it has tilted the balance toward those who believe there were several factors contributing to the demise of the dinosaurs.
``This shows definitively that a single asteroid impact or sudden series of eruptions might have been contributing factors but were not sufficient to cause extinction,'' Dr. Murali concludes.