Andrew C. Revkin, "Marine Life Leaped From Simple to Complex After Greatest Mass Extinction" (2006)

At least five mass extinctions, most presumably caused by asteroids that struck the earth, have transformed global ecology in the half-billion years since the emergence of multicelled life, lopping entire branches from the evolutionary tree and allowing others to flourish.

The greatest “great dying,” 251 million years ago, erased 95 percent of species in the oceans (and most vertebrates on land). But new research suggests that it was followed by an explosion of complexity in marine life, one that has persisted ever since.

Moreover, it happened quite suddenly, according to the study, which was led by scientists at the Field Museum of Natural History in Chicago and published in the current issue of the journal Science. The shift to complicated, interrelated ecosystems was more like a flip of a switch than a slow trend.

The researchers detected the change by analyzing records of marine fossils from 1,176 sites around the world, which are part of a new international archive, the Paleobiology Database (pbdb.org).

They found that marine life before the biggest global die-off, the Permo-Triassic extinction, was evenly split into two types of communities: simple ones, in which most species were anchored in place and got by without interacting with neighbors (like eating them or being eaten by them), and complex ones, with many interrelationships.

But since then, complex communities filled with grazers, scavengers, predators, burrowers and other mobile creatures have been three times as common as simple ones, said Peter J. Wagner, the lead author of the study.

The shift essentially took the oceans from a norm in which anchored (or sessile) creatures, including brachiopods and sea lilies, filtered food carried in currents to one dominated by roaming (or motile) fauna like snails, urchins and crabs.

Dr. Wagner said it was not clear why this particular extinction spasm had this permanent effect on the character of communities, while others did not.

A 2002 study led by Richard K. Bambach, an emeritus professor at Virginia Tech, found the general shift to a higher abundance of motile fauna from the early Triassic Period onward, but it did not examine patterns in individual communities.

But Dr. Wagner said motility was an enduring characteristic of the more variegated biological webs.

“The increased diversity of mobile species would have contributed to more complex ecological communities,” he said. “With sessile guys, everybody is just living next to one another and that’s it. With mobility and higher metabolism, you bump into each other more often, both literally and figuratively, and you end up with a greater number of potential interactions.”

Wolfgang Kiessling, a paleoecologist at Humboldt University in Berlin who assessed the study in an accompanying article in Science, said it represented “a major step forward,” particularly in finding a reliable way to distinguish simple ecosystems from complex ones through 500 million years of life history.