The fossils dated to a period before and during the ancient warming event known as Eocene thermal maximum 2 (ETM2), which occurred about 53 million years ago. During ETM2, temperatures increased between 5 and 9 degrees Fahrenheit (3 and 5 degrees Celsius) for about 100,000 to 120,000 years.
Tiny horses In particular, the team found that the first horse species, called Sifrhippus, which is "about the size of a small dog," got smaller by about 19 percent over time during ETM2, D'Ambrosia said. In comparison, during an earlier, longer warm period in the Eocene Period, those same horses decreased in size by about 30 percent.
"They're seeing a very similar response to what we're seeing in the [earlier warming event]," said Ross Secord, a vertebrate paleontologist at the University of Nebraska-Lincoln, who was not involved in the study but has studied the earlier warming event. "That's important: Now we have a predictable pattern."
Scientists aren't sure exactly why mammals would get smaller with warmer temperatures. Perhaps they simply need to be smaller to allow for more efficient cooling, Secord told LiveScience. (Smaller animals have higher surface area relative to volume to exchange heat with the air, which means heat is gained and lost faster.)