At least two ancient global warming events shrunk mammals, significantly decreasing their body sizes, researchers find
University of Michigan paleontologists have found that most mammals' body size significantly decreased in at least two global warming events that occurred in the past. They fear that a similar incident could take place owing to the drastic human-caused climatic changes.
Previously, researchers said that mammals decreased in size during a period of warming, called the Paleocene-Eocene Thermal Maximum (PETM), about 55 million years ago. This period lasted for approximately 160,000 years and temperatures rose by more than 14 degrees. However, new evidence unearthed by paleontologist Philip Gingerich and his colleagues revealed that mammalian "dwarfing" also occurred during a separate, smaller global warming event that occurred about 2 million years after the PETM, around 53 million years back. This period is known as Eocene Thermal Maximum 2 (ETM2) and lasted for 80,000 to 100,000 years. During this period, temperatures rose by up to 5 degrees.
"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," said Gingerich, a professor of earth and environmental sciences.
Researchers noted that this decrease in size is actually a common evolutionary response by mammals to extreme global warming events called hyperthermals. 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 researchers found that body size decreased during ETM2, but not as much as the dwarfism seen in PETM fossils.
"Interestingly, the extent of mammalian dwarfism may be related to the magnitude of the hyperthermal event," said team member Abigail D'Ambrosia of the University of New Hampshire. "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."
