Between 56 and 53 million years ago, Earth experienced a series of extreme global warming events that radically altered life on the planet.
During the first event, the Paleocene-Eocene Thermal Maximum (PETM), carbon levels spiked and temperatures increased as much as 8 degrees Celsius (14.4 degrees Fahrenheit). A second event, known as the Eocene Thermal Maximum 2 (ETM2), occurred two million years later and led to global temperature increase of about 3C (5.4F).
The ETM2 temperature increase might sound familiar. It's roughly the level of warming scientists suggest we may see if governments around the world don't de-carbonize their economies by reducing the amount of fossil fuel emissions they pump into the atmosphere.
One of the effects of those extreme climate changes millions of years ago was the dwarfing of mammals. New research, published in the journal Science Advances, suggests that not only do warmer temperatures shrink the body size of mammalian species, but the warmer it gets, the bigger the change.
The researchers, led by Abigail R. D'Ambrosia of the University of New Hampshire, examined the fossilized teeth of four different mammal types found within the Bighorn Basin of Wyoming, which lies about 100 miles east of Yellowstone National Park.
"We went out in the field every summer for several years and collected fossils," she said. "It turns out that, just like in humans, all mammals have this shiny, outer layer of our teeth, which is tooth enamel. And that stuff is really resistant to weathering and erosion over millennia, it turns out. So teeth tend to last longer than bone material out there. So we find a lot of fossil teeth out in the Bighorn Basin."
Tooth size also happens to be a great proxy for body size in mammals.
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What D'Ambrosia and her team found was significant. Not only did they see the dwarfing in two of the four mammals they were concerned with, they determined that the magnitude of dwarfing correlated to the magnitude of the climate extreme when it was contextualized with prior research on carbon and temperature increases in the deep past.
Previous research on Arenahippus pernix, an early horse, found dwarfing during the PTEM. D'Ambrosia and her team also found dwarfing in the species, but on a smaller scale during the less extreme ETM2.
"We find dwarfing, and we compare our horse to the PTEM horse, we see that dwarfing does seem to correlate to the magnitude of the event," she said. "So ETM2: Not as extreme as PTEM, and we see less dwarfing."
It's not clear, she added, if the body size decrease was due to temperature rise, increased carbon concentrations, or both.
"Nevertheless, it has to do with this change in the atmosphere, for sure," she said.