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April 20, 1995 Press Contact: Steve Koppes
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You Can Run, But You Can’t Hide: Mass Extinctions Spare No One

In the largest study of its kind ever performed, University of Chicago paleontologists David Jablonski and David Raup show that mass extinctions wipe out all types of life forms, without regard to special survival strategies they may have developed during times when extinction rates are relatively low. The results of the study will be published in the Friday, April 21, edition of the journal Science.

The two scientists studied the survivorship of 350 evolutionary lineages of marine mollusks –clams and other two-shelled ocean-dwellers– during the mass extinction event that killed the dinosaurs at the close of the Cretaceous Period, 65 million years ago. Marine fossils are the most abundant, widespread and well-studied of the organisms that make up the fossil record, and they are widely taken to be the best barometer for what happened to other kinds of living things. “What we found was that the end-Cretaceous event really was brutal,” said Jablonski, Professor of Geophysical Sciences. “It was an across-the-board extinction, and what we ordinarily think of as mechanisms for survival just didn’t work.”

Scientists had believed, for example, that larger-bodied organisms fared worse in mass extinctions than organisms that were simply smaller in size. Smaller organisms require less food, they rebound more quickly from disaster because their populations grow more quickly, and their numbers are larger to begin with (there are more mice than elephants, for example). But Jablonski and Raup found that for the 350 evolutionary lineages they studied, body size had no relationship to survival rate.

In addition, contrary to what previous studies have suggested, neither life habits nor feeding strategies had a bearing on survivorship. Species that burrowed through the sea floor–where they might be expected to be buffered from changes in ocean temperature or salinity–fared no better than scallops, oysters and other groups that dwelled on the sediment surface. Species that fed on the detritus of the ocean bottom did no better than species that filtered plankton from the water. Whether the organisms lived in deep or shallow water also had no bearing on their rates of survival.

The fact that organisms living in different environments–deep or shallow water, for example–had the same rates of extinction, “just blew our minds," Jablonski said. “This shows that there really was nowhere to hide.”

“This study leads us to believe that mass extinctions are really quite different from the ordinary evolutionary scheme of things,” said Jablonski. “They disrupt, rather than reinforce, the evolutionary patterns that we see at other times.”

The only factor that did seem to be a key to survival was widespread distribution. Organisms that were distributed over many continents were more likely to survive than those that had a narrower geographic range.

“This doesn’t tell us whether it was an asteroid or a volcano,” said Jablonski, “but it does constrain some of the killing mechanisms.” For example, it couldn’t have been something that simply deprived the deeper waters of oxygen, or killed off the plankton for a month, he said. “From the fossil record, we know there was a plankton crisis, and from that we would have thought that surface-feeders would have suffered more than detritus-feeders, but that was clearly not the case.”

He said, “One big question is whether our results explain mass extinctions in general. Maybe this change in the rules for extinction or survival helps to explain why the dinosaurs are gone and mammals weathered the end-Cretaceous extinction.

“There are some hints that other mass extinctions show the same kind of selectivity, but clearly much more comparative analysis is needed. Maybe this opens the door to a whole new way of looking at mass extinctions as an evolutionary force.”

Jablonski and Raup studied an 8-million-year slice of geologic time leading up to the end of the Cretaceous Period. “Unfortunately, this study doesn’t allow us to narrow down the time frame of that extinction event,” Jablonski said. “We don’t know if it happened over 5 million years or one bad weekend.”

This research was supported by grants from the National Science Foundation and the National Aeronautics and Space Administration.
Last modified at 03:50 PM CST on Wednesday, June 14, 2000.

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