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How global warming created tiny horses

About 56 million years ago, a massive release of carbon dioxide heated the earth's surface by about 10 degrees over 175,000 years. Many mammals responded to this by shrinking. 

By Stephanie PappasLiveScience Senior Writer / February 23, 2012

Artist's reconstruction of Sifrhippus sandrae (right) touching noses with a modern Morgan horse (left) that stands about 5 feet high at the shoulders and weighs approximately 1,000 pounds.

Danielle Byerley, Florida Museum of Natural History


An ancient global warming event shrunk the earliest horses down to the size of scrawny housecats, according to new research that could have implications for what mammals might look like in a future warming world.

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During what's known as the Paleocene-Eocene Thermal Maximum, or PETM, about 56 million years ago, a massive release of carbon into the atmosphere and oceans boosted average global temperatures by about 10 degrees Fahrenheit (5.5 degrees Celsius) over 175,000 years. Mammals responded to this climate change by shrinking, with about one-third of species getting smaller.

Now, new research reveals that these changes occurred in lockstep with temperature fluctuations during the period. The earliest-known horse Sifrhippus started out this period as a bit of a pipsqueak: The animals weighed only about 12 pounds (5.6 kilograms) on average, about the size of a miniature schnauzer.

But by 130,000 years into the PETM, Sifrihippus had shrunk down to only about 8.5 pounds (3.9 kg), the size of a small housecat. These humble horses then experienced a size rebound, bulking up to about 15 pounds (7 kg) in the last 45,000 years of this warm period, putting them on par with the average bichon frise.

Horse's mouth

Researchers 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 were able to track these changes over time using a trove of fossils from the southern Bighorn Basin in Wyoming. Measurements of horses' teeth found in the area suggested a distinct pattern of shrinking and growing body size over the PETM time period. At first, that finding seemed like a mistake, Bloch told Livescience, because the assumption had been that horses in the PETM were all uniformly teeny.

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