Geochemical analysis of fossils shows Florida’s ancient horses were homebodies

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Researchers examined strontium in 89 fossilized teeth excavated from two sites in Florida. Pictured is a tooth from the prehistoric horse Cormohipparion. Photo: Jenelle Wallace
Researchers examined strontium in 89 fossilized teeth excavated from two sites in Florida. Pictured is a tooth from the prehistoric horse Cormohipparion. © Jenelle Wallace

The major migrations seen in today’s wild equids were not a hallmark of the ancient horses who lived in Florida, according to researchers.

The findings suggest that Florida may have been a horse paradise five million years ago, providing everything they could want in a relatively small area.

This might explain their apparent lack of migratory habits, in contrast to today’s wild equines, including Burchell’s zebras in southern Africa which can travel as far as 300 miles and back as they follow the rains to green grass and water. The Mongolian wild horse, also known as the Przewalski’s horse, travel as much as 13 miles a day.

The findings of the study, published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, centered on research conducted by geologists in the University of Cincinnati’s McMicken College of Arts and Sciences.

They found that prehistoric horses in coastal Florida lived and died within a comparatively small area.

“It seems that these horses in Florida were relatively sedentary. They didn’t travel far distances,” said Jenelle Wallace, a graduate of the university and lead author of the study.

The study was the basis of her master’s thesis.

Today, she works as an engineering geologist for the New York State Department of Environmental Conservation.

A scene from the Miocene Period as an ancient species of horse, Parahippus, lower right, interacts with other carnivores and herbivores. Image: Jay Matternes, public domain, via Wikimedia Commons
A scene from the Miocene Period as an ancient species of horse, Parahippus, lower right, interacts with other carnivores and herbivores. Image: Jay Matternes, public domain, via Wikimedia Commons

“My third spoken word after mom and dad was horse. I’ve loved them ever since I was little,” she says.

The world’s first horses originated in North America. They lived there for 55 million years before spreading to Asia and Africa while going extinct on their home continent about 12,000 years ago.

The small three-toed animals lived like antelope, browsing leaves in deep forests. But during the Miocene Period between 23 million and 8 million years ago, horse evolution exploded into 15 different families. Horses developed bigger bodies, longer legs and hard hooves in place of toes to help them cover more ground.

Their teeth also changed, becoming bigger and longer for cropping coarse grass covered in abrasive silica dust instead of plucking soft leaves. It’s these teeth that helped University of Cincinnati geologists study how extinct horses lived.

They compared strontium isotopes found in fossilized horse teeth to the strontium in bedrock in different parts of the American Southeast to track the horses’ wanderings. Plants such as grass absorb strontium from the earth and the horses, in turn, absorb that strontium while grazing. In this way, strontium serves as a geographic marker.

Geology professors and study co-authors Brooke Crowley and Joshua Miller have used this technique to track the movements of other animals, both living and prehistoric. Crowley used bones collected from the nests of secretive goshawks to map the birds’ travels in Madagascar. She and Miller also are studying the movement of Ice Age mastodons in North America.

“There is a lot of opportunity for expanding the use of strontium to look at a variety of animal groups, time periods and locations,” Crowley said.

The study examined seven species of horse along with two known leaf-eaters: a prehistoric tapir and a distant relative of elephants called a gomphothere.

The results were surprising, researchers said.

Of all the animals studied, the tapir seemed to have the widest geographic range based on the high variability of strontium found in its teeth. But given that modern tapirs have relatively modest home ranges, researchers said it was more likely that prehistoric tapirs consumed river plants that absorbed nutrients carried far downstream.

Among the horses, the researchers found little variation in the size of their ranges. But the strontium showed a connection between horses and the sea. Like modern horses today in places such as Assateague Island National Seashore, prehistoric horses might have fed along the coast. Researchers suggested the vegetation horses consumed was influenced by marine-derived strontium from seaspray, precipitation or saltwater intrusion into groundwater.

“The study suggests we’re not the only couch potatoes. If animals don’t have to move, they won’t,” Miller said.

Migrating is a dangerous business, Miller noted. Animals face injury, illness and starvation when they travel great distances. And in the Miocene Period, horses had to outwit plenty of big predators such as saber-toothed cats.

“The energetic costs of moving are high,” Miller said.

Crowley, who also teaches in the university’s Department of Anthropology, said studies like this shed light on the habitat needs of animals long before they were influenced by human activities.

“Having a deep perspective is really important for understanding a species’ needs in conservation and management,” Crowley said. “If we just look at a narrow window of time — like 50 or 100 years — we don’t get a good picture of a species when it’s not in crisis.”

Using the geologic record, researchers can piece together how animals interacted, what allowed them to thrive and what ultimately caused them to perish, she said.

“By using this technique, we can answer questions in a way we couldn’t otherwise,” Crowley said.

“That’s the cool thing about geochemistry. You can unlock secrets in teeth and bones.”

The study was funded by grants from the University of Cincinnati Geology Department, Sigma Xi, the Geological Society of America and the American Society of Mammalogists along with the Association for Women Geoscientists’ Winifred Goldring Award.

Investigating equid mobility in Miocene Florida, USA using strontium isotope ratios
Jenelle P. Wallace, Brooke E. Crowley and Joshua H. Miller
https://doi.org/10.1016/j.palaeo.2018.11.036

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