Stumbling into a natural experiment
David's work on mass extinctions dates to his days as a student. For his Ph.D., David traveled up and down the East Coast of the U.S. looking at 65-80 million year old fossil mollusks in order to study background extinction the trickle of extinctions that go on constantly throughout Earth's history, which stand in stark contrast to catastrophic mass extinctions. Then in 1980, just as he was finishing this work, the paleontological community was rocked by the announcement of a bold, but promising, hypothesis: that the KT (Cretaceous-Tertiary) mass extinction, the event that eliminated virtually all dinosaurs 65 million years ago, was triggered by a massive asteroid slamming into Earth.
David realized that with his current research, he had a head start on collecting the data needed to look at this event more closely. He had stumbled onto a perfect natural experiment in his words "the perfect laboratory for comparing background and mass extinction." He could collect data on extinction patterns during the mass extinction and compare them to extinction patterns during the "normal" time that followed and what he discovered was a surprise.