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The online edition of the magazine published by The Johns Hopkins University, Zanvyl Krieger School of Arts and Sciences


Ice-Age Survivors

© 1998 Dr. Richard Paselk,
Humboldt State University
Natural History Museum

Earth scientists have for years been puzzled by a surprising tale of survival that occurred during one of the great ice ages, the period 330 million to 290 million years ago: How, they wondered, did so many major groups of marine life manage to avoid extinction?

Matthew Powell, PhD ’05, believes he has an answer. Research he conducted as a graduate student in the Morton K. Blaustein Department of Earth and Planetary Sciences led him to conclude that aquatic life forms that survived were specially equipped to tolerate severe variations in sea level and temperature. The others—less hardy, for a variety of reasons—died out in what Powell calls the “mass extinction that heralded the onset of the ice age.”

This research, published in the May 2005 issue of Geology, helps scientists understand what was occurring in the ocean millions of years ago, and it has implications for understanding the modern marine ecosystem, in which species are dying off at a rate that alarms many experts.

“Based on my research and that of other scientists, modern marine life ought to be relatively resistant to extinction. After all, those that survived the last ice age—like the brachiopods that I studied—should still be resistant,” says Powell, whose faculty adviser was professor Steven Stanley. “But look what’s happening: Fish populations are dwindling, coral reefs are dying, and more. From what we know, there should be many fewer extinctions occurring now than is average, and yet, there are more. The specifics of why this is happening now aren’t known in every case, though everyone’s pretty sure that humans are to blame. It may be that humans have altered the environment so much that we are now causing the extinction of species that should be relatively immune. We just don’t know.”

During his study, Powell scrutinized geographic patterns of evolution and extinction among brachiopods (simple, mollusk-like sea creatures) during a time when glaciers reached to within 35 degrees of the equator. (That means glaciers extended as far south from the North Pole as modern-day Memphis or Albuquerque and as far north from the South Pole as Buenos Aires.) He ended up with a complex database—the first of its kind—that tracked evolution and extinction rates according to latitude.

“According to my data, brachiopods that lived mostly near the equator suffered the highest rates of extinction and did not reappear in great numbers when the ice age was over,” Powell says. “That left the ocean populated almost entirely with those that lived over a wider geographic area.”

The brachiopods that lived near the equator died out, Powell believes, because they had not adapted to the annual temperature fluctuations they experienced due to the presence of glaciers. But brachiopods that lived far from the equator were accustomed to such temperature swings: They could survive and even thrive in both very cold and very warm seawater.

“Seasonality, which is the name we give to differences in annual high and low temperatures—increases as you move toward the poles,” Powell says. “Near the equator, they have almost no seasonality, as anyone who takes a winter vacation in the Caribbean can tell you.”

The study has whet Powell’s appetite for further research into the evolution and extinction of other marine species, including the ecological effect of the ice age’s onset. Now pursuing post-doctoral studies at Washington and Lee University, he says, “Anything that can kill off most of our planet’s life is cool—and terrifying.”

—Lisa DeNike





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