Regional Recovery More Rapid Following Late Ordovician Extinction

"We are currently in an undeniable biotic crisis," says Andrew Z. Krug, graduate student in geosciences. "We are not just interested in what will disappear, but what will reappear and when the recovery will take place."

During the Ordovician, the majority of life was found in the seas. Scientists consider climate change, specifically widespread glaciation, as the trigger for this mass extinction.

The researchers report in this week's on-line version of the Proceedings of the National Academy of Sciences, that "marine benthic diversity in Laurentia recovered to pre-extinction levels within 5 million years, which is nearly 15 million years sooner than suggested by global compilations."

Laurentia eventually became North America, however, during the Ordovician, it was located in the tropics and Pennsylvania was south of the equator. The researchers looked at the fossil record from Laurentia because large amounts of information are available in the Paleobiology Database (PBDB) sponsored by the National Science Foundation and housed at the National Center for Ecological Analysis and Synthesis.

"Laurentia is well studied and the fact that it was tropical suggests there should be a lot of diversity," says Dr. Mark E. Patzkowsky, associate professor of geosciences.

Previously, investigations of extinctions have been on a global scale and most used a global database developed by the late Jack Sepkoski of the University of Chicago. This database lists the first appearance of an organism and the last appearance of an organism.

"There is quantitative information missing from the global database," says Krug. "PBDB includes faunal lists, species occurrences and other information useful for standardization of sampling effort."

According to the global database, recovery from the Ordovician extinction took 15 to 20 million years.

"We suspected that there might be a sampling issue," says Krug. "We standardized sample size and looked at how diversity recovers."

The researchers looked at 35 million years from the Ordovician to the Silurian and divided that into seven approximately equal time periods. They assembled lists of taxa – groups of related organisms – that were then standardized to account for low fossil counts in time periods for which few fossil bearing rocks are easily accessible and high fossil counts in time periods where the fossil bearing rocks are easily accessible and frequently collected.

Comparing the raw data with the standardized data, Krug and Patzkowsky saw a large difference in the number of years necessary for recovery after the extinction. The raw data for Laurentia showed a recovery period of 10 million years while the standardized data showed only 5 million years for recovery.

"Based on other work, this suggests a good possibility that the region was operating differently than the globe as a whole," says Patzkowsky. "I would argue that the way the field considered the problem in the past was heavily influenced by the Sepkoski database. We show that at least in Laurentia, recovery was quicker than was thought globally."

Krug and Patzkowsky believe that the quicker recovery was caused by immigration of organisms from other areas of the globe. While this could account for the rapid rise of diversity after an extinction on a regional level, only an evolution of new organisms could account for a global diversity increase.

To see if other regions behave the same, Krug will look at faunal lists from Baltica – now Eastern Europe, Norway and Sweden – that was further south than Laurentia, Avalonia – now the United Kingdom and Nova Scotia – that was in a temperate area, and South Central Europe, which includes the western Mediterranean countries, that was even further south.

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NASA's Astrobiology Institute and NASA Exobiology supported this work.