By Cobb, K.
Science News , Vol. 161, No. 24
Sets of species may persist through major extinction events only to die off in the after-math, new research suggests.
Paleontologists recognize five cataclysmic episodes in Earth's history, times when 50 to 95 percent of existing species abruptly vanished. Scientists have long studied the causes and casualties of these mass-extinction events (SN: 2/24/01, p. 116). Recently, focus has shifted to survivors, but only those that went on to "fame and fortune," not those that later failed, says paleontologist David Jablonski of the University of Chicago.
While studying the aftermath of the Cretaceous-Tertiary, or K-T, extinction, which wiped out the dinosaurs 65 million years ago, Jablonski noticed that many surviving lineages of plants and animals lingered a paltry few million years and then petered out. Intrigued by this anecdotal evidence, Jablonski set out to test the hypothesis that a disproportionate number of survivors wind up dead in the recovery phase of a mass extinction. His results, presented in the June 11 Proceedings of the National Academy of Sciences, may have implications for the current biodiversity crisis.
Using a database of marine fossils, Jablonski considered two types of groups of related organisms, or clades--in this case, genera and orders. Genera are the smallest basic groups of related species; higher up on the taxonomic ladder, orders encompass hundreds of genera. Jablonski compared percent extinctions of these groups in the geologic stages immediately before and after each mass extinction. Geologic stages last 5 to 10 million years, the average life span of a single species.
Jablonski found elevated genera-extinction rates in four of the five postextinction stages and elevated ordinal losses in three. …