June 16, 2005

Fossils Tell the ‘Hole’ Story of Killer Drillers and Their Prey

ANN ARBOR, Mich. - In the classic textbook example, predators and prey are locked in an escalating evolutionary arms race. As prey develop defenses against their attackers, predators go them one better, devising new ways of penetrating their victims' defenses.

But it doesn't always happen that way, and investigating systems in which prey don't respond to predation by upping the ante can be as informative as studying the more typical sort, said University of Michigan paleontologist Tomasz Baumiller. In a report in the June 17 issue of Science, Baumiller and collaborators at Virginia Tech describe how they used fossil shells of animals called brachiopods to shed light on ancient ecological interactions.

Brachiopods, also known as lampshells, were plentiful in Earth's oceans 550 million years ago to 250 million years ago. Today the shelled creatures, which look something like clams, are still around but less prevalent. One way of assessing rates of predation or parasitism on brachiopods and other fossil shellfish is to look for round holes drilled into their shells by their enemies. The researchers studied more than 40,000 specimens from 18 museums, as well as field-based estimates from the literature.

"We wanted to determine at what frequencies drilling had occurred, whether that had changed through time, and what sorts of lessons we could learn from the patterns that we found," Baumiller said. What they found was that attacks on brachiopods were rare, but widespread and continuously present through the entire Paleozoic Era, suggesting that brachiopods were not locked in an arms race with drilling predators. Instead, predators probably picked on brachiopods only when they mistook them for something else or when there was nothing better to eat, the researchers believe.

"The reason is, they're not terribly nutritious---they have a relatively low amount of meat between their shells," which also may explain why you never see brachiopod chowder on restaurant menus, Baumiller said.

The researchers also found a slight increase in the frequency of drill holes on brachiopods in middle-to-late Paleozoic times and a second modest increase sometime after the end of the Paleozoic Era. Those findings hint at how the whole food chain was changing over time.

"These subtle increases in drill holes on brachiopods, from less than one percent in the early Paleozoic era to several percent today, suggest that predators have become increasingly abundant and active through the history of life," said Michal Kowalewski, an associate professor of geobiology at Virginia Tech. Over time, "the basic food supply in the oceans---phytoplankton---increased significantly, organisms up the food chain became more meaty, and predators---bony fish, snails and crustaceans---increased in frequency and diversified," he said. As predators specialized in attacking their shell-protected victims, prey groups also diversified and became better armored. As a result, the authors hypothesize, interactions between drilling predators and less desirable prey, such as brachiopods, became more frequent or more lethal.

"In a sense," Baumiller said, "we're using brachiopods as a dipstick to gauge the process of escalation."

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