Dwarfism In Mammals Also Occurred During Second Warming Period
November 3, 2013

Scientists Discover Second Instance Of Global Warming-Caused Mammal Shrinkage

redOrbit Staff & Wire Reports - Your Universe Online

While scientists have known for several years that some mammals became smaller during a period of warming known as the Paleocene-Eocene Thermal Maximum (PETM), researchers from the University of Michigan have found a second instance of mammalian “dwarfing” attributable to increasing temperatures.

During the PETM, which occurred approximately 55 million years ago, mammals such as primates and groups that include deer and horses became much smaller. Now, paleontologist Philip Gingerich and colleagues have found that a second, smaller global warming event, which happened two million years later, had a similar effect – and that discovery suggests a similar phenomenon may also occur as a result of modern-day, human-caused climate change.

“The fact that it happened twice significantly increases our confidence that we're seeing cause and effect, that one interesting response to global warming in the past was a substantial decrease in body size in mammalian species," explained Gingerich, a professor of earth and environmental sciences at the Ann Arbor, Michigan university.

Gingerich and scientists from the University of New Hampshire, Colorado College and the California Institute of Technology (Caltech) presented their findings Friday at the annual meeting of the Society of Vertebrate Paleontology in Los Angeles.

The study authors discovered decreasing body size appears to be “a common evolutional response” of mammals to extreme global warming events known as hyperthermals, and that these types of physiological changes could be “a predictable natural response for some lineages to future global warming.”

According to the researchers, the period known as the PETM lasted approximately 160,000 years, and during that period global temperatures rose an estimated nine to 14 degrees Fahrenheit at their peak. The second event, which occurred 53 million years ago, was identified as Eocene Thermal Maximum 2 (ETM2) and reportedly lasted between 80,000 and 100,000 years. It resulted in a peak temperature increase of about five degrees Fahrenheit.

“Teeth and jaw fossils of early hoofed mammals and primates that spanned this later climatic event were collected in Wyoming's Bighorn Basin, and the size of molar teeth was used as a proxy for body size,” the university said. “The researchers found that body size decreased during ETM2, but not as much as the dwarfism seen in PETM fossils.”

For instance, the researchers found that an ancient type of dog-sized horse known as Hyracotherium experienced a body-size decrease of approximately 30 percent during the PETM and 19 percent during EMT2. Following both events, the creature ultimately returned to its original pre-warming size, and the authors believe the magnitude of the warming event is likely related to the extent of dwarfism experienced by mammals.

Much of modern-day global warming is believed to have been caused by the burning of fossil fuels and the ensuing release of carbon dioxide and other greenhouse gases. The PETM and EMT2 events, on the other hand, could have been caused by the release of seabed methane clathrates, a kind of methane ice found in ocean sediments, Gingerich and his colleagues said. However, this is still the topic of ongoing research.

“Methane is a more potent greenhouse gas than carbon dioxide, and atmospheric methane is eventually transformed into carbon dioxide and water,” the university said, adding “the parallels between ancient hyperthermals and modern-day warming make studies of the fossil record particularly valuable.”

“Developing 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's climate,” noted Will Clyde, a member of the research team and an associate professor of geology at the University of New Hampshire.