Bridge of death.
The formation of the Isthmus of Panama caused a mass extinction, as revealed in these fossil shells.

(inset) Aaron O'Dea

A Lag Before Dying

As recent geologic events go, few had more dramatic consequences than the formation of a land bridge between Central and South America 3 million years ago. Formed by the Pacific tectonic plate sliding under the Caribbean one, the Isthmus of Panama divided the Atlantic and Pacific oceans, radically altering sea currents. A massive extinction of marine animals ensued, yet death may not have come quickly. According to a new study, the rate of these extinctions peaked about 2 million years after the Isthmus formed. The discovery complicates the tidy notion of an ecological change rapidly following an environmental one.

Before the Isthmus of Panama emerged, the open waters between North and South America were similar to the Pacific today. Both were muddy, loaded with plankton, and nourished by nutrient-rich waters that regularly rose from the depths. Once the Isthmus split the sea into the Caribbean and the Pacific, upwelling in the Caribbean stopped. The region's plankton starved, but its reef-building corals thrived. And, traditional theory goes, animals that loved the upwelling died out in their new environment within the geologically instantaneous time frame of thousands of years.

Not so fast, says marine paleontologist Aaron O'Dea, of the Smithsonian Tropical Research Institute in Balboa, Panama. He and colleagues tracked shifts in ancient ecosystems by counting the overall abundance of marine invertebrate fossils and analyzing how populations of species swelled or shrank over time. Reporting online this week in Proceedings of the National Academy of Sciences, O'Dea and colleagues conclude that after the Caribbean shifted from the Pacific to its present-day environment, many species that loved the old conditions managed to cling to life for as long as 2 million years before biting the dust. O'Dea says that these communities may have remained steady under deteriorating conditions until they reached a critical level, then collapsed. Extinction rates in the Caribbean peaked in the last 1 million to 2 million years, the team reports, with half the solitary, mud-loving coral species and a third of mollusk species dying out every million years.

The study "points out something we haven't noticed before: the time lag between environmental shifts ... and a peak in the extinction," says Peter Roopnarine, an invertebrate zoologist at California Academy of Sciences in San Francisco. Evolutionary biologist David Jablonski of the University of Chicago in Illinois says that the extinction delay is perhaps not surprising, given that hardier members of a species might be able to stick things out longer than the species as a whole. Whatever the cause, O'Dea says, the findings could mean that other "quick" extinctions may not have been so rapid after all.

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