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Evolution’s Gardener

USC College’s David Bottjer and his graduate crew dig up new clues about ancient mass extinctions.

By Eva Emerson
November 1, 2007

Evolution’s Gardener

Rocks collected from ancient seabeds clutter the worktables that dominate the center of paleobiologist David Bottjer’s lab. A grainy, yellowed one records the shape of a giant clam that lived 90 million years ago in what’s now Texas. Pieces of a 270 million-year-old limestone bubble in a bath of acid, revealing dime-sized shells long turned to stone. More rocks, carefully tagged, wait like unopened treasure boxes, each rife with fossils that could provide new views of life’s history.

An expert on animal evolution and rare fossils, Bottjer may be best known for his investigations of the more destructive aspects of evolutionary history — the mass extinction events. These planet-wide upheavals have punctuated the history of life on Earth at least five times in the past 500 million years. More than 99 percent of all the species that have ever lived have gone extinct, many, like the dinosaurs, killed off during one of the five big extinctions. Each cataclysmic event has severely pruned the tree of life, re-routing evolution and shaping the make-up of the globe’s current biotic citizenry.

Understanding mass extinction events has gained a new urgency in the face of today’s environmental problems — from climate change and dwindling numbers of rare species to pollution and large-scale habitat destruction. These problems, and the attendant rapid loss of biodiversity, have led some to speculate that humans now stand in the throes of the planet’s sixth great extinction.

In the last few years, Bottjer, a professor of earth sciences and biological sciences in USC College, has made substantial progress in his work to explain the extinction that took place 250 million years ago at the end of the Permian period. Although not as well known as the extinction that led to the disappearance of the dinosaurs 65 million years ago, the end-Permian killed off more than 90 percent of all species and ranks as the largest known extinction in the planet’s history.

“This was when animal life faced its biggest crisis since it first evolved,” he said. “People used to think of it happening suddenly, like a car hitting a wall. But our studies show millions of years of environmental stress, more like a lingering chronic illness.”

He theorizes that increased volcanic activity led to global warming, which set off a chain of widespread environmental change and a protracted era of environmental stress.

“We want to understand the consequences of a major environmental crisis like this for perspective on our current environmental crisis,” said Bottjer. “Our studies also provide a fascinating window onto evolution. What happens after the playing field is leveled and opportunities for new species to expand and thrive are widely available?”

Bottjer, who brings the same thoughtful intensity to training graduate students as he does to his research, credits much of the recent progress to the diligent work of a “batch” of exceptional graduate students in his lab. Under his mentorship, they have grown into full-fledged scientists and gone on to successes of their own.

A long, long time ago
Behind a desk in Zumberge Hall covered in neat, towering piles of journals, books and papers, Bottjer leans back in his chair. He’s talking about his summer trip to fossil beds in Nevada and British Columbia, but his thoughts are about 250 million years away, considering the condition of the globe at the close of the Permian period and the start of the Triassic.

“These were catastrophic times,” he said. “And very interesting times. The Triassic begins with a mass extinction and ends with another substantial extinction. The intriguing thing is that these extinctions are about 50 million years apart. That’s pretty close.”

Along with many other scientists, Bottjer believes that both extinctions were caused by an extended period of severe global warming. “We’re finding that the two extinction events probably are related. They’re both associated with times of great volcanism, global warming and stagnating, low-oxygen oceans,” he said.

This was during the age when the land masses, driven together by plate tectonics, were fused into the supercontinent known as Pangaea. Tectonic activity, foreshadowing Pangaea’s breakup, led to volcanic eruptions that covered hundreds of square miles with molten rock.

The volcanism, the theory goes, led to extreme global warming, decreasing the difference in seawater temperatures between the polar regions and the equator. Ocean circulation, normally driven by these differences, slowed and the oceans stagnated. The supply of oxygen dwindled. Marine creatures began to show signs of stress.

With so little oxygen, Bottjer thinks anaerobic microbes might have come to dominate deep ocean niches. These include sulfate-reducing bacteria that create hydrogen sulfide and carbon dioxide as a byproduct of metabolism. Hydrogen sulfide, which is often said to smell like rotten eggs, is very toxic to most animal life. The stinky, toxic water might have killed off large groups of animals in deep water and slowly, over five to 10 million years, crept upwards into the mid-ocean and coastal areas and reefs, where it devastated life.

His team’s research on the fossil record is helping to refine what’s known about life long ago and these mass extinctions. It’s also lending strong support to his theory.

Bottjer and graduate student Catherine Powers published this fall a report in Geology, a top journal in the field. The fifth-year doctoral student’s studies of bryozoans, a group of tiny marine invertebrates that live in colonies, reveal how their distribution and species diversity shifted over hundreds of millions of years. Looking at fossils and data from around the globe, she showed that bryozoans followed the pattern described in Bottjer’s theory — deep water species began to disappear about 260 million years ago, well before the end-Permian extinction, while shallow water species declined gradually until, at the time of the crisis, very few were left.

“It turns out that bryozoans may be a good ‘canary in the mine’ for detecting these stressful periods,” Bottjer said.

For insight into the first signs of environmental change, Bottjer and former student Matthew Clapham, who earned a Ph.D. at USC last year and will join the faculty at the University of California, Santa Cruz, in January, looked back earlier in the fossil record.

In a paper published July 30 in the online edition of the Proceedings of the National Academy of Sciences, they reported that ecological changes associated with the extinction were already starting 10 million years before the end-Permian crisis. Analyzing rock samples from Nevada, China and Greece, Clapham identified 24 fossilized marine communities that lived in the mid-Permian (270 to 260 million years ago) or the late-Permian (260 to 252 million years ago).

He examined more than 33,000 individual fossils to determine the relative abundances of mollusks and brachiopod species in each period. He found that while brachiopods dominated the communities in the mid-Permian, as has been thought, the mollusks steadily increased. Bottjer and Clapham discovered that, by the late-Permian, mollusks and brachiopods lived in mixed communities, and in some cases, mollusks overtook brachiopods in abundance. The finding contradicts the older belief that the shift was abrupt and tied to the mass extinction.

“We think the ecological shift was caused by environmental stress,” Clapham said, “and that the mollusks, as a group, were better able to tolerate the increasingly deteriorating conditions.”

Immediately after the end-Permian extinction, the sea floor, once home to a diverse flora and fauna, looked nearly bare. Some mollusks, however, not only survived the crisis, but thrived. Bottjer and former student Margaret Fraiser documented this in a paper published in the July issue of Paleobiology, with the attention-grabbing title of “When Bivalves Took Over the World.”

Fraiser, now an assistant professor at the University of Wisconsin, Milwaukee, showed that the bivalves (a group of mollusks that includes clams, oysters, scallops and mussels) tolerated many of the environmental changes of the early Triassic. Then, their populations expanded.

According to Bottjer, they showed that “a few bivalves — a few clams — could tolerate these conditions, and no one else could, so they went bananas. In the rocks from this time, you start finding zillions of bivalves. We figure they had some physiological way to tolerate the changes.”

The idea that stagnating oceans led to an accumulation of poisonous hydrogen sulfide has been strengthened in studies by another of Bottjer’s graduate students, Pedro Marenco, in work with Frank Corsetti, associate professor of earth sciences in the College. Marenco uses geochemical techniques to examine sediments in rocks that date to the early Triassic. He has found telltale signs of bacterial sulfate reduction in the rocks. Marenco, who completed his degree this past summer, is now a postdoctoral fellow at UC Riverside.

One of the few men in the College to receive a grant from USC’s Women in Science and Engineering program (WiSE), Bottjer received $25,000 to help fund the work of four recent female graduate students — Powers, Fraiser, Nicole Bonuso and Sara Pruss. Pruss, who graduated in 2004, did a postdoc at Harvard before landing a faculty position at Smith College. Bonuso is now an assistant professor at California State University, Fullerton. The WiSE grant enabled the women to pursue studies at locales around the world and fueled their success.

“We’ve got as good a group of students as anywhere,” Bottjer said. “And this batch has done very well.”