American Scientist

To the Editors:

Three general problems complicate attempts to determine the true time of the rudists' disappearance using data from the fossil record ("The Rise and Fall of Rudist Reefs," March–April): Not all reefs are fossilized, not all fossilized reefs are discovered, and dating fossils involves uncertainty. In the case of the late Cretaceous, there is also a specific problem in that falling sea levels may have led to systematic reef destruction via exposure and weathering. The most reasonable interpretation of the apparent disappearance of the rudists less than a million years prior to the Cretaceous-Tertiary boundary is that it is an artifact resulting from incomplete data, and that they actually went out together with the dinosaurs. To believe otherwise one must posit that by coincidence, two very rare extinction processes occurred nearly simultaneously due to independent causes, which is an improbable scenario.

Alan Wolfe
San Francisco, California

Dr. Johnson replies:

The causes of major extinction events tend to be controversial. The terminal Cretaceous demise of rudist bivalves and dinosaurs is no exception. It is true that fossil dating involves uncertainty and that falling sea levels may have led to systematic reef destruction, but coral-algal reefs and carbonate platforms continued to develop across the extinction boundary in several areas of the world. My article implies that the extinctions of both the rudists and the dinosaurs occurred prior to the meteorite impact that struck the Earth in the Yucatan Peninsula. It is possible that the rudists actually went out with the dinosaurs due to a single causal mechanism. However, we have yet to conclude on one mechanism that accounts for the extinction of all the groups that died out at the Cretaceous-Tertiary or other major extinction boundaries.