When Sifrhippus, the earliest known horse, first appeared in the forests of North America more than 50 million years ago, it would not have been mistaken for a Clydesdale（强健的挽马） . It weighed in at around 12 pounds -- and it was destined¹ to get much smaller over the ensuing（接着发生的） millennia². Sifrhippus lived during the Paleocene-Eocene Thermal³ Maximum, a 175,000-year interval⁴ of time some 56 million years ago in which average global temperatures rose by about 10 degrees Fahrenheit⁵, caused by the release of vast amounts of carbon into the atmosphere and oceans.

About a third of mammal species responded with 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 smallhouse cat (about 8.5 pounds) in the PETM's first 130,000 years and then rebounded⁶ 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 phenomenon in mammals during this period, and new 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 in Gainesville offers new evidence of the cause-and-effect relationship between temperature and body size. Their findings also offer clues to what might happen to animals in the near future from global warming.

In a paper to be published in the Feb. 24 issue of the international journal Science, Secord, Bloch and colleagues used measurementsand geochemical composition of fossil mammal teeth to document a progressive decrease in Sifrhippus' body size that correlates very closely to temperature change over a 130,000-year span.

Bloch, associate curator of vertebrate（脊椎动物的） paleontology（古生物学） at the Florida Museum of Natural History, said multiple trails led to the discovery.

One was the fossils themselves, recovered from the Cabin Fork area of the southern Bighorn Basin near Worland, Wyo. Stephen Chester, then an undergraduate student at Florida, now an anthropology⁷ Ph.D. candidate at Yale and a co-author on the paper, had the task of measuring the horses' teeth. What he found when he plotted them through time caught Bloch and Secord by surprise.

"He pointed⁸ out that the first horses in the section were much larger than those later on," Bloch recalled. "I thought something had to be wrong, but he was right -- and the pattern became more robust⁹ as we collected more fossils."

A postdoctoral researcher in Bloch's lab for the first year of the project, Secord performed the geochemical analysis of the oxygen isotopes¹¹ in the teeth. What he found provided an even bigger surprise.

"It was absolutely startling when Ross pulled up the first oxygen isotope¹⁰ data," Bloch said. "We looked at the curve and we realized that it was exactly the same pattern that we were seeing with the horse body size.

"For the first time, going back into deep time -- going back 10s of millions of years -- we were able to show that indeed temperature was causing essentially¹² a one-to-one shift in body size within this lineage（血统，家系） of horse. Because it's over a long enough time, you can argue very strongly that what you're looking at is natural selection and evolution -- that it's actually corresponding to the shift in temperature and driving the evolution of these horses."