An evolutionary group has a significantly better chance of surviving for a long time in the geologic record if it first appears right after a mass extinction.
University of Cincinnati geologist Arnold Miller will present his findings Tuesday morning Nov. 6 during the annual meeting of the Geological Society of America in Boston.
Professor Miller used a database of marine fossil genera compiled by J. John Sepkoski to examine longevity trends throughout the Phanerozoic (the last 540 million years). In four separate cases, he found that genera first appearing following mass extinctions survived for longer periods of time, on average, than those that first appeared at other times.
"There was already a sense that organisms originating in the wakes of mass extinctions were generalists with respect to their geographic and environmental distributions," said Miller. "My analysis indicates that these characteristics promoted evolutionary longevity."
Miller said that the trend is apparent no matter what the ultimate cause was of each mass extinction. Genera that were more widespread, might have fared better over the long run because of a kind of "safety in geography." If a catastrophe decimated the individuals living in one region, then a genus could still survive if individuals belonging to the genus also lived in other regions.
To conduct his analysis, Miller divided the Phanerozoic into 156 "bins" or substages. Then, he looked at the average longevity of genera originating in each bin. Significant peaks in mean longevities occurred in the substages following major mass extinctions in Late Permian, Late Triassic, and Late Cretaceous -- three of the "big five" extinctions of the Phanerozoic -- and following a lesser, but still significant extinction at the end of the Jurassic.
"These are very sharp peaks," noted Miller, who followed up his first analysis with a number of statistical techniques to weed out artifacts in the data set. "I was trying and trying to kill the pattern, but it wouldn't go away."
One enigma in the analysis is that the pattern does not extend back into the Paleozoic, the earliest of the three eras that comprise the Phanerozoic. Although there were fairly high extinction rates during parts of the Cambrian, Ordovician and Devonian, Miller's analysis showed no clear relationship between extinction events and longevity in any of those periods. "To see nothing is quite something," he said, summing up that intriguing finding.
It is possible that, after the Paleozoic, there was a major change in the dynamics of evolution, but Miller noted that any real explanation for the difference between the Paleozoic and post-Paleozoic remains to be determined.
Miller is currently working with a team of geologists worldwide to build an online database that depicts the occurrences of marine genera throughout the Phanerozoic, and which will incorporate data on the geography and paleoenvironment of each occurrence.
In the future, he hopes to use these data to assess directly whether the longer-lived genera really were those with wider geographic and environmental distributions. "We really haven't looked definitively at the characteristics of the post-extinction players, but with the databases we're building, we'll be able to."
UC Department of Geology
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