Miniature horses roamed a much hotter Earth 50 million years ago, scientists say

The animals shrank over tens of thousands of years to adjust to temperature

WASHINGTON — More than 50 million years ago, the Earth was a hotter place than it is today and horses the size of pet cats roamed the forests of North America, U. S. scientists said on Thursday.

These earliest known horses, known as Sifrhippus, shrank over tens of thousands of years in order to adapt to the higher temperatures in a period when methane emissions spiked, possibly due to major volcanic eruptions.

The research could have implications for how the planet’s modern animals may adapt to a warming planet due to climate change and higher carbon emissions, scientists said.

Researchers made the discovery after analyzing horse tooth fossils uncovered in Wyoming that showed the older ones were larger, and that the species had shrunk over time.

Many animals became extinct during this 175,000- year period, known as the Paleocene- Eocene Thermal Maximum, some 56 million years ago. Others got smaller in order to survive with limited resources.

“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

Are animals going to be able to keep up and readjust their body sizes over the next couple of centuries?

ROSS SECORD

UNIVERSITY OF NEBRASKA- LINCOLN

actually corresponding to the shift in temperature and driving the evolution of these horses,” said co- author Jonathan Bloch of the Florida Museum of Natural History.

Average global temperatures rose by about 15.5 C during that span due to massive increase in carbon that was unleashed into the air and oceans.

Surface sea temperature in the Arctic was about 23 C, much like the temperatures of contemporary subtropical waters today.

The research showed that Sifrhippus shrank by almost one third, reaching the size of a small house cat ( about four kilograms) in the first 130,000 years of the period.

Then, the horses grew larger again, to about seven kilograms in the final 45,000 years of the period.

About a third of known mammals also minimized themselves during this time, some by as much as one half.

“This has implications, potentially, for what we might expect to see over the next century or two, at least with some of the climate models that are predicting that we will see warming of as much as four degrees C over the next 100 years,” said co- author Ross Secord of the University of NebraskaLincoln.

Some birds have already been observed to have become smaller in size than in cooler times in the past, he said.

However, the forecast changes are expected to happen over the next century or two due to a boost in carbon emissions dating back to the start of the Industrial Revolution.

Millions of years ago, the climate change happened much more slowly, taking 10,000 to 20,000 years to get five degrees hotter, he added.

“So there’s a big difference in scale and one of the questions is, ‘ Are we going to see the same kind of response?’ Are animals going to be able to keep up and readjust their body sizes over the next couple of centuries?”

The findings are published in the journal Science.