Change in size of earliest horses driven by climate change

A study in Science suggests that temperature was the most likely factor driving decreases in the body size of the earliest horse, during a global warming event 56 million years ago.

When the earliest known horse, Sifrhippus sandae, first appeared in the forests of North America more than 50 million years ago, it weighed around 12 pounds – and it was destined to get much smaller over the ensuing millennia.

Sifrhippus lived during the Paleocene-Eocene Thermal Maximum (PETM), a 175,000-year interval of time some 56 million years ago in which average global temperatures rose by about 10 degrees Fahrenheit. Scientists say the warming was caused by the release of vast amounts of carbon into the atmosphere and oceans.

About a third of mammal species responded with a significant reduction in size during the PETM, some by as much as one-half. Sifrhippus shrank by about 30 percent, to the size of a small house cat–about 8.5 pounds–in the PETM’s first 130,000 years, then rebounded to about 15 pounds in the final 45,000 years of the PETM.

Scientists have assumed that rising temperatures or high concentrations of carbon dioxide primarily caused the “dwarfing” phenomenon in mammals during this period. The research, led by Ross Secord of the University of Nebraska-Lincoln and Jonathan Bloch of the Florida Museum of Natural History at the University of Florida, offers evidence of the cause-and-effect relationship between temperature and body size. 

The researchers used measurements and geochemical composition of fossil mammal teeth, recovered Wyoming’s Bighorn Basin, to document a progressive decrease in Sifrhippus’ body size that correlates very closely to temperature change over a 130,000-year span.