Q: How do we know the extinction of the dinosaurs wasn't due to a worldwide pandemic, either bacterial or viral?

A: Many scientists believe that a major asteroid or comet impact near the Yucatan Peninsula in Mexico caused sufficient firestorms and dust clouds to cause many types of dinosaur to disappear from the fossil record. But that could be just coincidence, says paleontologist Joe Skulan, an assistant faculty associate in the department of geology and geophysics at UW-Madison.

"There is absolutely no way to be sure that the extinction of the non-avian dinosaurs wasn't caused by an epidemic, or by anything else," says Skulan, who adds that not all dinosaurs went extinct about 65 million years ago; birds descended from surviving dinosaurs.

"All extinctions are caused by the same thing: the failure of members of a group of organisms to reproduce faster than they die, and explaining why this happens is difficult," he says.

Even the much more recent extinctions of North American mammoths and mastodons, which happened only 10,000 years ago "can be surprisingly difficult to explain," Skulan adds. "If we don't understand that, what are the odds we ever will have the kind of information necessary to explain what happened 65 million years ago? Nil."

But people have a powerful desire to understand why so many types of dinosaurs suddenly died out, he adds. "People want answers, and scientists are only too happy to supply them." But achieving certainty would go "far, far beyond anything empirical evidence can and ever will be able to supply."

The impact hypothesis seems reasonable because it happened very close to the time the non-avian dinosaurs went under, but it's impossible to know for sure, Skulan concludes. "It's not very satisfying, and it's possibly, or even probably, wrong, but it's the only sort of answer that is possible."

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