Turning Over an Old Leaf

Stacy Tibbets
September 05, 2006

"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.

fossil series
Peter Wilf

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.

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 themin 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 onlyknow 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 Wilf
Department of Geosciences

Peter Wilf

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."

Freshly dug plant fossils at Laguna del Hunco.
Peter Wilf

Freshly dug plant fossils at Laguna del Hunco.

New clues to extinction

In 2000, Wilf decided to bring quantitative techniques to bear on a classic paleontological problem—the asteroid strike that occurred 65 million years ago at what geologists call the K-T boundary. This massive impact and its subsequent atmospheric effects are now widely believed to have caused the extinction of the dinosaurs.

Wilf first encountered the K-T boundary via the studies of Kirk Johnson of the Denver Museum of Nature & Science, who had demonstrated that a massive plant extinction occurred at the same time as that of the dinosaurs. Starting in the late 1980s, Johnson collected over 22,000 specimens to document his findings, and in 2002, Wilf, Labandeira, and Johnson graded most of them for insect damage. This work provided the first conclusive evidence of a wide-scale insect extinction at the same time.

Wilf, Johnson, and colleague Brian Huber next used paleoclimatic analysis techniques on the entire collection. Their study documented a brief warming period 200,000 to 300,000 years before the asteroid hit, followed by a cooling about 100,000 years later, both correlating to the marine fossil record of that time. But this temperature spike had little influence on plant or animal species diversity, they concluded.

A further analysis of the floral turnover across the K-T boundary itself, however, added significant knowledge to what was known about the plant extinction at the time of the asteroid impact. It revealed the dramatic and immediate loss of at least 60 percent of plant species and a comparable number of the insects that ate them. "These effects are long-lasting, through the Paleocene era," Wilf says.

In Patagonia

In 1997, Wilf recounts, Kirk Johnson made a trip to Argentina's Patagonia region, and while there briefly visited a site that was something of a legend to paleobotanists: the ancient crater lake named Laguna del Hunco. Fossil plants were first discovered there in the 1920s, but the number of specimens taken had been small, and they had been somewhat unscientifically collected. The site, located seven hours by land from the nearest city, had been visited only sporadically since then. Given only 45 minutes to canvas the area by his Argentinian guide, Johnson spent the time photographing specimen after specimen of beautifully preserved fossils.

After seeing Johnson's photos, Wilf immediately began writing research grants to study the area. An award from the
National Geographic Society enabled him to begin work at the site in 2002. In 2004, he was awarded $500,000 through the NSF Biodiversity Surveys program, and in the year following he received both the John T. Ryan, Jr., Faculty Fellowship from the Penn State College of Earth and Mineral Sciences
and a $625,000 Packard Fellowship, a "bolt from the blue" award that gives him great latitude in how the money can be used. On several ensuing visits to Patagonia, Wilf and his collaborators have begun to excavate, thoroughly collect, and canvas for the first time what he calls "one of the most diverse fossil floras ever, from any time period, anywhere."

"Laguna del Hunco is an amazing white and black landscape," Wilf explains. "It's a white lakebed full of fossils—and black volcanic rocks. A huge eruption, probably in the early Eocene, created a caldera, which sagged afterwards, forming a lake. The animals and plants then were deposited in this small lake in the side of the caldera."

At this remote site, and at two others nearby—Río Pichileufú and the Salamanca Formation—exists a previously unknown diversity of ancient plant life dating from the Paleocene and Eocene epochs. "It's just completely outrageous," Wilf says, "because you're walking along and you see fossils on the ground everywhere—fish and frogs and insects too. I've seen two of my colleagues literally break down into tears there because they couldn't handle it."

Already Wilf's team has identified over 186 new species. "We have more than enough specimens right now," he says, "to write high-profile papers in very high-quality journals for a long time without ever having to collect another fossil. Everything about it is important, because the deep-time history of South America is very poorly studied, compared with that of Europe or North America."

"Particularly in terms of South American insects and insect damage, there's never been anything close to what we're studying at Laguna del Hunco. South America today has one of the most, if not the most diverse insect faunas in the whole world—but for the last 65 million years, the entire paleontological insect record consists of about 80 species. The entire insect damage record consists of one or two illustrations. Everything we're doing there is new."

"That region is very different from the rest of the world, geographically and botanically," he adds. Though today Patagonia is arid, shielded from Pacific moisture by the Andes mountains, at the time these specimens were deposited, 62 to 47 million years ago, the region was still connected to Gondwana, a massive southerly supercontinent that included parts of present-day Antarctica, Australia, and South Africa. The Andes didn't arise for many millions of years, Wilf notes, so the region was probably sub-tropical and wet.

The discoveries at Laguna del Hunco indicate conclusively and for the first time that South America has a long history of high plant diversity. But because of the poor fossil record, evolutionary connections to the plants of modern South America have been elusive. "We're actually having trouble finding a really good example of something that's alive today in the South American tropics that resembles what we're finding," Wilf says. "We're discovering instead that the flora at Laguna del Hunco is more closely related to what's now in northeastern Australia, New Caledonia, and other areas of the southwest tropical Pacific"—regions once part of Gondwana. Patagonia is part of this big ancient ecosystem, but until now there hasn't been a really effective large-scale effort to find out what these plants are, how old they are, how many there are, and what ate them."

The team is currently working on identifying its massive finds. "We have people working on the fish, the frogs, the plants, the insects, and the insect damage on leaves," says Wilf. Specimens from the 25 principal quarries at Laguna del Hunco are trucked to the Egidio Feruglio Paleontological Museum in Trelew, on Argentina's east coast. There, paleontologist Rubén Cúneo, co-principal investigator on the project, and other project participants analyze, photograph, and store the specimens. But the roads become impassible during the frigid Argentinian winter, which prevents work in the field during North American summers.

Wilf and his colleagues also face a severe lack of contextual history, partly because collecting fossils in the tropics, where the groundcover is so dense and topsoils are so thick, is notoriously difficult. "There are huge gaps to fill," he admits. But we can say that South America in general had a lot of diversity, and we can document that diversity back at least 55 million years."

Researchers in the field at Laguna del Hunco, Patagonia, Argentina.
Peter Wilf

Researchers in the field at Laguna del Hunco, Patagonia, Argentina.

At home with fossils

The summer of 2006 found Wilf characteristically busy, attending research conferences in Bilbao, Spain and Beijing. This November he'll be back in Patagonia, collecting plant fossils from the late Cretaceous era. Another ongoing project involves correlating the numerous leaf characteristics he and others have identified with the "phylogeny of angiosperms,"—that is, the evolutionary tree of flowering plants.

But it may be here in the Smithsonian's storage rooms that he is most at home. As we walk through the narrow passageways, Scott Wing hails him in friendly tones, and a couple of graduate students seem eager to see him as he stops to check on their work. Then, over the course of a long summer afternoon, he briskly tours me from case to case, room to room, revealing the museum's hidden treasures: spiral-shaped arthropod fossils the size of manhole covers; fossil boles of ancient conifers that once stood in present-day Maryland; inked mylar drawings, created from field sketches, of plant-insect interactions from Laguna del Hunco; and thousands of red-tinted glass slides containing one of the best existing collections of "cleared leaf" specimens—modern leaves that have been bleached and stained to make their vein patterns more visible.

My time is almost up, but the indefatigable Wilf beckons me to yet another row of towering archival cabinets, eager to show me one more fossil collection to which he has contributed. As I protest gently, citing the rush-hour traffic that awaits outside the museum, he pauses and smiles indulgently.

"Why," he asks at last, "would you ever want to leave this place?"

Peter D. Wilf, Ph.D., is assistant professor and John T. Ryan, Jr. Faculty Fellow of geosciences in the College of Earth and Mineral Sciences; pdw3@psu.edu. Some of the work described above was funded by the National Science Foundation. In addition, in 2005 Wilf was awarded a Packard Fellowship for Science and Engineering from the David and Lucile Packard Foundation. Data from the Laguna del Hunco project will eventually be made public through the NSF-funded Paleobiology Database and a bilingual Penn State-hosted Web site.

Last Updated September 05, 2006