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Staying put. Sedentary ocean-dwellers aren't pushovers.

Incumbent Critters Hard to Defeat

When it comes to marine life, reorganizing the neighborhood often requires razing the town. Many new species pop up in the aftermath of mass extinctions, but not all mass extinctions cause a sea change: A new study finds that like often replaces like, and that only three of history's five big extinctions were drastic enough to give the newest--and often most complex and adaptable organisms--the foothold they needed to dominate the oceans.

Paleontologists Richard Bambach, Andrew Knoll, and the late John Sepkoski sorted Sepkoski's vast catalog of marine fossils into two categories: ones that swam, and ones that stayed put or passively floated. Even though the number and type of animals in each category varied through time, the ratio of mobile to immobile creatures has only shifted three times in history, they report in the 13 May issue of the Proceedings of the National Academy of Sciences.

After an extinction 440 million years ago, floaters and animals glued to rocks and sea bottoms dominated until the global wipe-out 250 million years ago at the end of the Permian period. After the great Permian extinction, mobile sea-dwellers gained ground (so to speak), and within 20 million years, the ratio was about 50:50. After the mass extinction that finished off the dinosaurs 65 million years ago, the fleet-finned dominated the sedentary by 3 to 1. Smaller mass extinctions before and between the critical extinctions didn't upset the balance.

Bambach and his colleagues say this discovery supports the idea of ecological incumbency: Just as a political newcomer may struggle to unseat an inept four-term senator, even highly efficient and adaptable species have trouble toppling old kings from the hill. This interpretation was supported by a second analysis the team did with another coarse division: animals with and without the sophisticated physiology, like circulatory systems and gills, necessary to buffer them from changes in the make-up of seawater. Here, too, the more advanced organisms took the same slow and stuttering path to dominance.

"It's a major advance," says paleontologist Peter Sheehan, head geology curator at the Public Museum of Milwaukee. "It's basically what we've been expecting to see" with changes in the ecosystem occurring after major extinction events, he says.

Related sites
Richard Bambach's Web site

List of Mass Extinctions (marine and terrestrial)