Climate Change Drove Shrinkage In Ancient Horse
February 24, 2012

Climate Change Drove Shrinkage In Ancient Horse

The ancient sifrhippus, the earliest known horse, lived around 50 million years ago. It was very distinct in its appearance because it was only about the size of a modern day house cat, weighing in around 12 pounds.

The horse lived in what is known as the Paleocene-Eocene Thermal Maximum (PETM), a 175,000 year period where the Earth's atmospheric temperature rose by about 10 degrees Fahrenheit, caused by a great release of carbon into the atmosphere and oceans.

In response to the increase in greenhouse gas, one-third of mammal species decreased in size. The sifrhippus decreased by 30 percent to about 8.5 pounds during the first 130,000 years of the PETM event, it then increased to over 15 pounds during the final 45,000 years of the climate change.

Researchers have found a direct correlation between the change in the temperature and the change in the size of the sifrhippus.

The 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, studied the geochemical composition of the horse's teeth to document the decrease of the horses body size through the geologic timeframe of the PETM.

The fossils were recovered from Cabin Fork, located near the southern Bighorn Basin near Worland, Wyoming. Stephen Chester, an undergraduate who measured the teeth, caught Bloch and Secord by surprise with the findings.

Bloch said: “He pointed out that the first horses in the section were much larger than those later on. I thought something had to be wrong, but he was right – and the pattern became more robust as we collected more fossils.”

Secord, who was a postdoctoral researcher in Bloch´s lab for the first year of the study, studied oxygen isotopes in the teeth and also found a big surprise. According to Bloch, “It was absolutely startling when Ross pulled up the first oxygen isotope data. We looked at the curve and we realized that it was exactly the same pattern that we were seeing with the horse body size.”

The findings, according to Secord, raise questions as to how plants and animals respond to changes in the climate, including the changes in today´s climate. Scientists predict that the Earth´s temperature is expected to rise about 7 degrees Fahrenheit over the next couple of centuries, due to a 40 percent increase of atmospheric carbon dioxide since the start of the Industrial Revolution. During the PETM the temperature changed over a 10,000 to 20,000 year timeframe to become 10 degrees hotter.

The researchers question whether animals will be able to adapt quick enough due to the increased temperatures. According to what is known as Bergmann´s rule mammals of a given genus or species grow smaller in hotter climates.

The change in temperature is only part of the equation, Secord also reports that increased carbon dioxide decreases a plants nutrition level, which may cause animals to be smaller due to poorer nutrition.

The researchers published their findings in this week's issue of Science.


Image Caption: An artist's reconstruction of a modern horse compared with Sifrhippus. Credit: Danielle Byerley, UFL


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