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Graduate student Abigail D'Ambrosia (at right) and geology professor Will Clyde

When the Earth heated up about 50 million years ago in a series of extreme global warming events, early mammals shrank in response. While this mammalian dwarfism had been previously linked to the largest of these events, 91制片厂 doctoral student Abigail D鈥橝mbrosia and colleagues connected this dwarfism to a second, smaller, so-called hyperthermal, indicating an important pattern that could inform our understanding of current human-caused climate change.

The findings created a buzz when D鈥橝mbrosia presented them at the Society of Vertebrate Paleontology鈥檚 annual meeting in Los Angeles earlier this month.

Scientists have known that during the largest of these hyperthermals, known as the Paleocene-Eocene Thermal Maximum (PETM), temperatures rose an estimated 9 to 14 degrees Fahrenheit and mammal size shrank. D鈥橝mbrosia wanted to build on this knowledge, which her advisor, 91制片厂 professor of geology Will Clyde, put forth in his doctoral dissertation.

鈥淲e wanted to find out if this was a repetitive pattern,鈥� says D鈥橝mbrosia. 鈥淚f we knew that, maybe it would help us predict what would happen in a warming event today.鈥�

The repeat warming-and-shrinking cycle gives scientists greater confidence that this phenomenon is in fact cause and effect, and that this decrease in mammals鈥� body size might have been in result to the warming.

The researchers collected teeth and jaw fossils of two early hoofed mammals 鈥� Hyracotherium, an early horse the size of a small dog, and Diacodexis, an ungulate 鈥� and the primate Cantius in the fossil-rich Bighorn Basin region of Wyoming. 鈥淭o paleontologists, the Bighorn Basin is littered with fossils. If you walk around with a keen eye, you start seeing all these little tiny teeth,鈥� says D鈥橝mbrosia, who has done field work there since she came to 91制片厂 in 2010.

Using the size of molar teeth as a proxy for body size, the researchers found that the mammals鈥� body size diminished during a second smaller hyperthermal, called the ETM2. Hyracotherium decreased about 19 percent in size, Diacodexis about 20 percent, and Cantius about 8 percent.

鈥淚nterestingly, the extent of mammalian dwarfism may be related to the magnitude of the hyperthermal event,鈥� D鈥橝mbrosia says. During the ETM2, which lasted 80,000 to 100,000 years, about half as long as the PETM, temperatures rose an estimated 5 degrees Fahrenheit. During the warmer PETM, which occurred about 2 million years earlier than the ETM2, Hyracotherium had a body size decrease of approximately 30 percent.

Body sizes rebounded following both hyperthermal events.

Because both the PETM and the ETM2 hyperthermals coincided with increased levels of carbon dioxide in the atmosphere, this work could have implications for current climate change study.

鈥淒eveloping a better understanding of the relationship between mammalian body size change and greenhouse gas-induced global warming during the geological past may help us predict ecological changes that may occur in response to current changes in Earth鈥檚 climate,鈥� says Clyde.

For D鈥橝mbrosia, this work and the attention it鈥檚 drawing is the realization of childhood fantasies. 鈥淚 knew I wanted to be a paleontologist ever since I was a kid,鈥� she says. The Easthampton, Massachusetts, native took a route to 91制片厂 that let her sample collegiate styles: She began at her local Greenfield Community College, finished her undergraduate degree at Smith College, then came to 91制片厂 for her master鈥檚 and doctoral degrees.

鈥淚t turns out that Clyde鈥檚 research interests perfectly matched my own, including topics in paleontology, stratigraphy, and climate,鈥� she says of her decision to come to 91制片厂 after several years in the workplace. The many opportunities for travel and fieldwork that Clyde鈥檚 research offered were an additional draw. 鈥淢y time as a master's degree student at 91制片厂 was so fulfilling that I have stayed on for my Ph.D.鈥� she adds.

In addition to D鈥橝mbrosia, Clyde, and Gingerich, researchers on the project are Henry C. Fricke of Colorado College and Kathryn Snell of the California Institute of Technology. The research was funded by the National Science Foundation (EAR0958821), the Geological Society of America, the Paleontological Society, and Sigma Xi.

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