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Looking way, way back for insights on a warming planet

What Hyopsodus, Diacodexis, and Nanomomys might teach us about climate change

An artist's rendition of Eocene
primates. Source: Smithsonian

June 2012-- Around 55 million years ago, a thumb-sized primate called Nanomomys thermophilus made its home in the Bighorn Basin of northwestern Wyoming. Now a region of dry and barren badlands, back then it was a lush landscape covered in ferns, vines, and nut trees, crisscrossed by rivers and streams and as balmy as today's subtropics.

Some of the first modern mammals roamed this ancient land, including Diacodexis, a deer-like hoofed animal; Hyopsodus, similar to a weasel; Hyracotherium, a horse the size of a house cat; and various pint-size primates such as the diminutive Nanomomys.

Today, paleontologist Ken Rose possesses all that remains of Nanomomys: a few tiny, tiny teeth, each about a millimeter in length and easy to mistake for a crumb of dirt. They are just a few of the more than 1,000 fossils that he and his colleagues have collected during a decade of field work at their Wyoming site.

The scientists are using the Bighorn Basin assemblage to document the appearance of these first modern mammals and to understand how a dramatic episode of global warming called the Paleocene-Eocene Thermal Maximum (PETM) shaped their evolution. Like today's climate change, the PETM was driven by a surge in atmospheric carbon dioxide.

It was very analogous to what we're seeing now: a climatic event of rapid global warming brought on by changes in carbon isotope ratios in the atmosphere," says Rose, a professor at the Johns Hopkins Center for Functional Anatomy and Evolution. The episode lasted 200,000 years, during which temperatures rose 5 to 9 degrees Celsius, and had profound effects on mammal populations. "We'd like to know which orders succeeded and which went extinct, and why," says Rose.

While Eocene mammals have been found in many North American sites, as well as in Europe, Egypt, and Asia, conditions in the Bighorn Basin were just right for preserving fossils. As a consequence, says Rose, "the record in this basin is the best in the world for Paleocene and Eocene mammals."

The Museum of Paleontology at the University of Michigan recently published a monograph written by Rose and his colleagues that documents the team's decade of research on the PETM's animal life, which uncovered more than 1,000 specimens representing more than 50 mammal species including a new genus and five new species.

Rose does field work in the Bighorn Basin each summer, where he joins paleontologists from several other research institutions. "We collect in many different ways," he says. "Most is surface prospecting: You walk the outcrops; then you pick up anything you find within a one- or two-meter area. If you find a concentration, it will warrant digging and screening."

Nanomomys teeth
Nanomomys teeth

Rarely, if ever, do the scientists find an intact skeleton. "You'll find something that looks like someone put a bomb in a skeleton and blew it up: Here's a piece of femur, a humerus, a jaw," says Rose. "Then you have to determine how it goes together." It's like doing a four-dimensional jigsaw puzzle, he says, with the fourth dimension being deep time, the millions of years that have elapsed since the Eocene. "We don't necessarily know what the animal should look like since it may have no close relatives today."

Tools of the paleontological trade still include the standard ice pick, rock hammer, and shovel. But during his career, says Rose, a few new implements have enhanced his research.

One is a low-tech innovation, a superfine mesh screen used in a procedure called screen washing. In this technique sediment is placed inside a set of nested wooden boxes that each have a screen bottom of progressively finer mesh. The sediment is sifted and washed in the nearby Bighorn River. Fine sediment filters through the mesh leaving coarser sediment, rocks, and fossils on top. Rose used to use a coarser mesh screen until a colleague persuaded him to try the superfine (0.5-millimeter) mesh. Although more laborious, it captures minute specimens that would otherwise evade notice, such as the 1-millimeter Nanomomys teeth.

The researchers scoop the dust-like material remaining on the screen into coffee cans and take it back to the lab. There they examine the powder-like concentrate under a microscope one-half teaspoon at a time. It can take an hour to review five teaspoons, 100 hours to get through an entire coffee can's worth. But while the procedure is laborious, a can of dirt may contain hundreds of fossils.

The other tool that has made a radical difference in Rose's science is a far more sophisticated device, a $2 million micro CT scanner. Housed at Penn State, which purchased it for crystallographic studies, the specialized CT scanner can reveal details not visible to the naked eye or even discernible through a standard microscope. Images of the Nanomomys teeth, for instance, show every groove and crevice, every cusp and crease. The shape of the molars, in particular—broad with deep basins and sharp cusps—suggested that the animals were omnivorous, able to crush seeds, and chew on insects. Such intimate details convinced Rose that Nanomomys is clearly a new species, one that belongs to a group of extinct mammals called the microsyopids. It was probably a mouse- or shrew-sized primate that spent a good part of its life in the trees.

The CT scan images also enabled Rose to measure precisely each dimension of the Nanomomys teeth, figures he then plugged into a statistical model that relates body size to tooth dimensions. The calculation showed that the diminutive animal weighed as little as 8 grams, or about a quarter of an ounce. "You could put four of them in an envelope and mail it with one stamp," says Rose. No living primate is as tiny. The smallest today, the mouse lemur of Madagascar, weighs 30 grams or more.

In fact, dwarfing appears to have been one consequence of a warming planet. The research team found many other examples of mammals that evolved to smaller sizes during the PETM. These include Hyracotherium, the dawn horse, which was about the size of a house cat. A principle called Bergmann's rule explains why such shrinking occurs. According to this rule, smaller animals can dissipate heat more readily than large animals because they have a higher surface area-to-volume ratio. So, small stature is an asset in a warm environment.

The scientists also found that the mammals from the Bighorn Basin do not gradually appear in the fossil record.

"They show up instantly," says Rose, which indicates that they did not evolve in the basin but migrated there. "Warming surely had something to do with it," says Rose. "It is generally thought that higher temperatures expanded the tropical/subtropical zones toward the poles, opening northern connections between the continents, which were much closer then. This would have allowed easier dispersal of organisms from Asia or Europe to North America."

The epoch eventually proved calamitous for some species, such as Nanomomys. But others flourished, and we can see their descendants today in mammals such as the horse, rhino, tapir, pig, hippo, giraffe, and primates like ourselves.

So what can this long-ago span of warming teach us about present-day environmental flux? How much is translatable?

Rose notes that human activity obviously did not contribute to the surge of atmospheric carbon at the dawn of the Eocene. Scientists believe that the carbon came from vast undersea stores and that some type of cataclysm—such as a volcano, comet, or tectonic shift—triggered its release. But the consequences were the same: a greenhouse effect and rising global temperatures. Rose also points out that today, atmospheric carbon levels and the planet's temperature are increasing much more rapidly than they did during the PETM.

"One thing we may speculate is that as temperatures rise, some animals may fare well and some may not," he says. "It's quite likely we'll lose species. We're already seeing that with the loss of amphibians."

Studying an ancient period of global warming might help inform our understanding of present-day climate change, says Rose. But there is still much to learn about that past event. So this summer, he'll be back in Wyoming for another season of digging, picking, and sifting through the sediment.

--Melissa Hendricks