Recipe for Extinction:
Add Species and Mix Thoroughly

One June day about 65 million years ago, an asteroid nearly six miles across detonated off what is now the Yucatan Peninsula. The dust it kicked up plunged the planet into a prolonged winter; acid rain gave lakes and streams the chemistry of a battery; firestorms cremated terrestrial plant and animal life.

This end of the Age of Reptiles sounds like one of the biblical catastrophes favored by 19th-century creationists. But it followed from an observation by geologist Walter Alvarez in the late 1970s that there was an unusual concentration of the rare element iridium in sediments marking the end of the Cretaceous Period.

Twenty years later, paleontologists are highly skeptical, although the theory remains popular in the media.

Fossil-mammal expert David Archibald is impatient with any claims that don't jibe with the fossil record. An asteroid is "an equal-opportunity killer," he says, but the great Cretaceous die-off was quite selective. Frogs and salamanders, which should have been most vulnerable to acid rain, survived unscathed.

Archibald favors what sounds like an old idea: dropping sea levels destroyed dinosaurs' coastal stomping grounds. The updated difference, Archibald says, is that modern conservation biology explains how populations decline when patches of habitat shrink and fragment. And shrinking habitat isn't just a guess anymore; recent studies calculate that at the end of the Cretaceous, the continents gained as much dry land as the entire present area of Africa.

Paleontologist Robert Bakker, of Casper College in Wyoming, cites evidence of dinosaur mass migrations allowed by changing sea levels--a displacement that spells ecological chaos. Today there are many examples of how introduced species are seriously disrupting ecosystems. "When living things are suddenly transported from one continent to another, all hell breaks loose," Bakker says. "You're releasing new pests, predators, competitors."

Though tempting, this comparison is "unscientific," according to Archibald, because it can't be tested. Since the plant fossil record does show a sudden extinction, plausibly caused by a reduction in light, Archibald favors a scenario that retains at least a supporting role for an impact in finishing off the dinosaurs after ten million years of decline.