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A warming climate eons ago appears to have been responsible for shrinking animal species, says a University of Nebraska-Lincoln scientist whose findings have implications for a hotter future.

Rising temperatures during a 175,000-year hot spell that occurred some 56 million years ago likely caused the earliest known horse to shrink by as much as a third, says the study published Thursday in the journal Science.

The study examined the evolution of horse ancestor Sifrhippus during the Paleocene-Eocene Thermal Maximum (PETM), a 175,000-year period in which average global temperatures rose by about 10 degrees, brought on by a massive release of atmospheric carbon.

"Now we're seeing the same degree of warming happen possibly over a century or two," said Ross Secord, assistant professor of Earth and atmospheric sciences at UNL and curator of vertebrate paleontology at the University of Nebraska State Museum.

Secord, with University of Florida-Gainesville Professor Jonathan Bloch and others, analyzed the size and chemical makeup of fossilized mammal teeth to document the decrease in Sifrhippus' body size closely linked to climate warming.

About a third of all mammal species shrank significantly during the PETM, some by as much as half. Sifrhippus shrank from about 12 pounds to 8.5 pounds, the size of a small house cat, during the first 130,000 years of the PETM.

Bloch said researchers were surprised when measurements of prehistoric horse teeth demonstrated they declined in size over time during much of the period.

Secord performed the geochemical analysis of oxygen isotopes in the teeth and uncovered an even bigger surprise.

"It was absolutely startling when Ross pulled up the first oxygen isotope 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, he said, they could show that, indeed, temperature was causing essentially a one-to-one shift in body size within this lineage 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," Bloch said.

Secord said the finding raised important questions about how plants and animals will respond to rapid change as scientists predict climate warming by as much as 7 degrees over the next century. Those predictions are based largely on the 40 percent increase in atmospheric carbon dioxide since the start of the Industrial Revolution.

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During the PETM, Secord noted, temperatures rose over a period of 10,000 to 20,000 years.

Increased temperatures are not the only changes animals will have to adapt to, he said. Greenhouse experiments show that increased atmospheric carbon dioxide lowers the nutritional content of plants, he said, which could have been a secondary driver of dwarfism during the PETM.

Ornithologists, Secord said, already have started to notice there may be a decrease in body size among birds.

Might some modern mammals also shrink like they did millions of years ago?

"If I had to venture a guess," Secord said, "I would say they will."

Reach Kevin Abourezk at 402-473-7225 or kabourezk@journalstar.com.

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