"It's a really striking, surprising pattern," Jablonski said. "And it appears that other animals and plants were playing the same game, even on land," now that previous studies are looked at with new eyes.

The three paleontologists began working on the problem more than a decade ago. The first step involved completing a massive standardization of all living and many fossil bivalve species to ensure their consistent and proper classification.

To accomplish the task, Jablonski churned through stacks of monographs, some dating back to the 19th century, and combed drawer after drawer of bivalve specimens in the Smithsonian Institution and other natural history museums in Chicago, London, Brussels, Belgium; and Leiden, the Netherlands.

The forces behind the flood of evolutionary activity that flows from the tropics remain a mystery. "But now that we have a handle on the dynamics that set up this spectacular planet-sized gradient, we can begin to get at the underlying processes in a whole new way," Jablonski said.

Jablonski, Roy and Valentine will attempt to address this and related questions as they push their analysis further back in time.