In the fossilized evidence of an insect's lunch over 50 million years ago, Peter Wilf can read the story of ancient climate change.
—By Stacy Tibbetts
"What I think is cool is looking at these really early ones," Peter Wilf says, reaching for a three-foot-wide specimen drawer labeled 1878. "Here you have the collections from the original explorations of the Western U.S. Territories—when the Smithsonian was just starting, and before that, actually," he enthuses. "These are reference collections from the original federal agency that explored the American West."
We are in the inner sanctum—the paleobotanical "type room" in the East Wing of the Smithsonian National Museum of Natural History in Washington, D.C. Here rest the finest specimens of plant fossils in all of North America. The small room is well-lit, unlike the cavernous storage vault full of towering cabinets that surrounds it. A researcher at a side table peers through a microscope. The largest dictionary I've ever seen sits on a stand nearby.
Wilf, assistant professor of geosciences at Penn State and a Smithsonian research associate, leads me across the room. He pulls out a drawer that contains a magazine and a flat grey rock the size of a dinner plate. At six-foot-eight, he towers over the cabinet, and his large hands dwarf the fossil specimen as he gently removes it. This particular one, of a ginger leaf, shows the feeding traces of a now-extinct beetle, known as a rolled-leaf hispine. It is one of only eleven known samples that contain evidence that this creature ever existed. Wilf made this discovery in 1999 and named the beetle Cephaloleichnites strongi, after Donald Strong, a researcher who had studied the beetle's latter-day equivalent. The magazine, which turns out to be a copy of the journal Science, documents Wilf's discovery.
I squint at the rock and the magnified magazine photos, trying to make the connection. "Don't feel bad if you haven't actually seen it yet," Wilf says. He carries the rock over to an unused microscope. "I'm going to show you right where it is."
As I stare into the glass, a tiny vertical scratch appears, about three-eighths of an inch long and ending at a leaf vein. The surface of the scratch has tiny lines running across it, perpendicular to its length. "You see that the veins of the leaf go like this," Wilf indicates, "and the trail of the beetle is perpendicular to the veins. So as it inches along the leaf, its mandibles scoop inward and make the tiny marks. And the veins aren't tasty—it doesn't like them. So it scoops between the veins and then goes to the next area, and the next one."
The meal he's describing took place 53 million years ago. It's a particularly old example of "niche conservatism"—a close relationship between two organisms that persists across an evolutionary timescale. As Wilf notes, the modern descendants of the rolled-leaf hispine beetle still feast on the ginger plant. His discovery pushes back the start of that relationship by some 30 million years.
An ancient fossil leaf...shows a single line of beetle larvae feeding damage less than one centimeter long...consisting of mandible marks between leaf veins.Credit Peter Wilf
A larger context
Wilf's patience for such subtle, small discoveries—and the large insights they yield—has already begun to elevate him into paleontology's elite. In only eight years since earning his Ph.D., he's pioneered a new branch of paleobotany, showing how insect-feeding patterns on fossil plants tell the story of ancient climate change. In a field traditionally dominated by extremely selective sampling, he's demonstrated the efficacy of performing massive quantitative studies. He's improved the fundamental tools for fossil-leaf identification, and helped amass some of the largest plant fossil collections in existence. And since 2004, he's been principal investigator at one of the most promising paleobotanical dig sites in the world today, at Laguna del Hunco in Argentina.
Wilf's doctoral studies, in the department of geology at the University of Pennsylvania, were supervised by Scott Wing, current chair of the department of paleobiology at the National Museum of Natural History. "So I started immediately working out of here," Wilf notes. "I liked Scott's approach. He put paleobotany in a larger context. The field can be very inward looking—but Scott collaborates with paleoclimatologists, marine geologists, vertebrate paleontologists, and others. This, I thought, is the kind of open approach I want to take—whatever I do, I want it to be useful to other fields."
For his thesis, Wilf studied the climate change during the boundary between the Paleocene and Eocene time periods, about 55 million years ago and 10 million years after a giant asteroid smashed into what is now the Yucatan Peninsula in Mexico—the strike that most scientists now believe caused mass extinctions on Earth. Working from a well-known Paleocene dig site in southwestern Wyoming, he and Wing, with Chris Beard, a vertebrate paleontologist from Pittsburgh's Carnegie Museum, eventually reconstructed the entire ecosystem of that time period, including the mammals, plants, and sedimentology.
"Our core methodology," Wilf says, "was to get large samples that wouldn't be biased by collecting hundreds, thousands of leaves, and identifying every one. We call it 'censusing,' and in the process, we very carefully describe every single specimen that we find—the plant identification, the stratigraphy or layer in which the fossil is found, the size of the fossil, and any insect damage.
"In that respect, we differed from most paleobotanical studies, whose goal is to identify new ancient species and place them in the family tree of plants." For these studies, he notes, researchers typically collect only specimens of the type they're studying, and within that type only the best examples. Such a selective sample is no good for analyzing relative abundance or diversity, he says. "Our large collections are better for researching change over time or making statements about large-scale trends."
Although Wilf is not the first to census fossil floras, he has become known for doing so on a large scale and for applying the technique to insect damage on leaves. "Being a geologist in this field gives you the perspective of stratigraphy—the study of rock layering—and the tradition of quantifying things, looking for large-scale patterns in the data," he notes. "Botanical reconstruction of a species by itself won't tell you if there was a mass extinction at a certain point in history. You only know that by looking at all the plants, and by putting them in stratigraphic order."
What insects tell
Wilf has also worked to address one of paleobotany's basic challenges: leaf identification. "Unlike collecting dinosaurs," he explains, "almost every time you go out collecting fossil plants, you find something new." In 1999, he and several colleagues wrote and self-published a comprehensive guide, the Manual of Leaf Architecture, and are now working on a system that other scientists can use to quickly categorize and "score" leaf fossils and the visible insect-feeding damage on them. A pending journal publication will add hundreds of high-quality leaf illustrations to this effort.
Taking innovative—and even maverick—stances is nothing new for Wilf. While still a graduate student, he challenged the accepted method for using fossil plant leaves to estimate paleoclimates, a complicated system based on 30 leaf characteristics. He suggested instead a system based entirely on analysis of the leaf margin. A jagged edge, he and his colleague Dana Royer have since shown, helps a plant to survive in colder weather by increasing the rates of photosynthesis and transpiration at the beginning of the growing season when temperatures are limiting. "I jumped headlong into a massive and fractious debate with my first paper," he remembers with a smile. "That paper is still my most-cited single piece of research."
Peter WilfCredit department of geosciences
Combining this interest in paleoclimates with censusing would eventually draw Wilf into an area of paleontology that previously had been almost completely unexplored. With paleoentomologist Conrad Labandeira, the Smithsonian's curator of fossil arthropods, he began exploring the effects of climate change on insect damage on fossil leaves.
"We had a very clear hypothesis, based on modern ecology, which is that there is global warming, and that it's reflected regionally in the flora," Wilf says. "But what about insect damage? Does that increase with climate change? You'd suspect that it would, since there's more insect feeding on modern plants in tropical climes. But no one had ever looked at the fossil record. We did, and the answer seems to be 'yes'."
A 1999 paper in Science documented the team's findings. Combining their analysis based on fossil insect damage and leaf size and shape with the presence in the fossil record of plants such as palms, ferns, and gingers, and heat-loving reptiles including crocodiles, they posited a substantial temperature increase beginning at 55 million years ago that turned what is now the western United States into a moist, subtropical forest.
Because it suggested a new way of estimating global climate change, the study received considerable attention from the popular media. But Wilf discounts the analogy to present-day global warming. "The effects of this warming period were real," he says, "but the time span in which they occurred stretched over millions of years, and the changes that took place involved the forces of evolution. You can't make easy comparisons between that and our current brief global-warming trend."