March 4, 2011

Early Horses Had Different Diet

A well-established theory that horses evolved through natural selection has been verified with a groundbreaking study of fossil records by two anatomy professors at New York College of Osteopathic Medicine (NYCOM).

Early ancestors of the modern horse likely ate fruit, the researchers noted Thursday, after studying horse teeth fossils dating back 55 million years.

As land conditions evolved over time, the diets of horses became more mixed and their teeth became tougher allowing for chewing and digestion of grasses that may have had gritty dust or soil mixed in, said the researchers, whose study appears in the journal Science.

The evolution of larger, sharper molars closely follows historical changes in climate, but with a large enough gap between environmental shifts and dental changes to suggest that many horses died off along the way, according to the team.

"We found that evolutionary changes in tooth anatomy lag behind the dietary changes by a million years or more," said co-author Matthew Mihlbachler of the New York Institute of Technology.

"One of the advantages of studying extinct creatures like prehistoric horses is we can look at how animals responded to their environments over millions of years -- something that biologists who study living species cannot do," he said.

Mihlbachler worked with fellow researcher Nikos Solounias to examine fossilized teeth of 6,500 horses from 222 different populations of more than 70 extinct species. They also compared the data to records of climate change in North America over time.

Surprisingly, while some of the extinct populations showed signs of extremely abrasive diets, Mihlbachler said it seemed like horses back then had it rather easy, suggesting that "strong natural selection" for different types of teeth only occurred occasionally during brief intervals in horse history.

Solounias helped to develop a process known as dental mesowear analysis to reconstruct the diets of extinct species by measuring food-related wear and tear on fossil teeth. He and Mihlbachler used the process to probe study wear patterns on the molars of thousands of fossil teeth. They then analyzed the data against records of North American climate changes they would have shifted animal diets from rainforest fruits to more abrasive grasslands.

"Lag time in the evolution of horse teeth in comparison to dietary changes is critical," Mihlbachler explained. "We found that evolutionary changes in tooth anatomy lag behind the dietary changes by a million years or more."

"The earliest horses from (about) 55.5 million years ago had short crowned (brachydont) molars with poorly developed shearing crests, suggesting a frugivorous (fruit-based) diet," the study said.

As grasslands became more predominant, horse teeth grew larger and taller with sharper edges. "High-abrasion mesowear patterns resembling those of modern horses and zebras have persisted for the past four to five million years," it said.

While horses have been labeled classic examples of evolution through natural selection by many paleontologists, the theory has been difficult to test because most horse species are now extinct.

"'You are what you eat': we hear this all the time, but now we know it is true," explained Thomas Scandalis, Dean of NYCOM. "This study shows that the evolutionary path of horses as we know them today was affected by the food available to their prehistoric ancestors."

Mihlbachler and Solounias's research shows that not only has the number of horse species been greatly reduced in the past few million years, but also that their diets have been increasingly restricted.

"Living horses are anything but typical examples of the dietary ecology of this once great group of mammals," said Solounias.

The research also suggest that larger and more evolved teeth indicate greater likelihood of survival.


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