Washington: Mammal body size decreased significantly during at least two ancient global warming events millions of years ago, a new study has found.
A similar outcome is possible in response to human-caused climate change, according to a University of Michigan paleontologist and his colleagues.
Researchers have known for years that mammals such as primates and the groups that include horses and deer became much smaller during a period of warming, called the Paleocene-Eocene Thermal Maximum (PETM), about 55 million years ago.
Now, Philip Gingerich and his colleagues have found evidence 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 ago.
"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 concluded that decreased body size "seems to be a common evolutionary response" by mammals to extreme global warming events, known as hyperthermals, "and thus may be a predictable natural response for some lineages to future global warming."
The PETM lasted about 160,000 years, and global temperatures rose an estimated 9 to 14 degrees Fahrenheit at its peak.
The smaller, later event analysed in the latest study, known as ETM2 (Eocene Thermal Maximum 2), lasted 80,000 to 100,000 years and resulted in a peak temperature increase of about 5 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 researchers found that body size decreased during ETM2, but not as much as the dwarfism seen in PETM fossils.
For example, the study revealed that a lineage of early horses the size of a small dog, called Hyracotherium, experienced a body-size decrease of about 19 per cent during ETM2.
The same horse lineage showed a body-size decrease of about 30 per cent during the PETM. After both events, the animals rebounded to their pre-warming size.
The study was presented at the annual meeting of the Society of Vertebrate Paleontology in Los Angeles.