Pony down: Did ancient warming shrink the earliest horses? Scientists are already noticing other animals getting smaller with contemporary climate change. Image: /Danielle Byerley/Florida Museum of Natural History
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The first horses in North America would not have been able to hold their own in the Triple Crown. At just about 5.6 kilograms the Sifrhippus sandrae hoofed onto the scene some 56 million years ago about the size of a small dog.
But then a funny thing happened. In the next 130,000 years during the Paleocene–Eocene Thermal Maximum, these small equines got even smaller, reaching the tiny size of 3.9 kilograms—some 30 percent lighter than their initial heft. Just 45,000 years later, however, the genus had bulked up to seven kilograms. And the horses were not the only ones. Many other mammals in the area followed the same pattern.
These animals' sizes likely resulted from relatively rapid climate change, suggest the authors of a new study published online Thursday in Science.
The study "highlights the importance of temperature on evolution—particularly mammal evolution," says Felisa Smith, a professor of biology at the University of New Mexico in Albuquerque, who wrote an essay on the findings in the same issue of Science. And it adds a new high-resolution tracking of body size and temperature during a crucial—and long puzzling—time in geologic history.
View a slide show of the visual history of ancient miniature horses
Looking the small horse in the mouth
The researchers did not have complete skeletons to measure for all of the animals, so to track the size of the horses over time they looked at their teeth—in particular, their molars. "It turns out that teeth are much better than femurs," Smith says. A leg bone "does tell you something about size, but teeth are much better." And as far as teeth go, she says, "the best thing to know is the area of the first molar."
The teeth came from a fossil-rich area called Cabin Fork in Wyoming and are part of a substantial collection at the University of Florida built in part by study co-author Jonathan Bloch, an associate curator of vertebrate paleontology there. From the collection, the research team could estimate the size of about 44 diminutive adult horses.
Some 40 percent of other mammals in the area seem to have experienced similar shrinking and subsequent growth, notes co-author Ross Secord, a vertebrate paleontologist at the University of Nebraska–Lincoln. They stuck with the small horses, however, because they had much more solid records from which to accurately date the samples.
The researchers used oxygen isotopes left by freshwater in the fossils to track mean annual temperature from when the animals had been alive. In particular, they sampled the isotopes from teeth of a large, water-dwelling mammal Coryphodon. With these isotope readings, "you get a little, tiny window as to what the temperature was at that time," Smith says.
This close reading has excited Smith and others who have been tracking animal size over the ages. "Although we knew that temperature might set a maximum for body size," Smith says, the new findings actually present a mechanism—and do so in a very detailed manner, showing "how animals responded to a particular temperature at a particular place at a particular time."