WASHINGTON - Early ancestors of the modern horse likely ate fruit that did not require sharp molars to grind down, a study of horse teeth fossils dating back 55 million years has shown, scientists said Thursday.
As land conditions evolved over time, horses' diets became more mixed and their teeth became tougher to be able to chew and digest grasses that may have had gritty dust or soil mixed in, said the study in the journal Science.
The evolution of bigger, sharper molars closely follows historical changes in climate, but with a large enough gap between environmental shifts and dental changes to suggest that plenty of horses died off along the way, the research said.
"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."
Mihlbacher and colleague Nikos Solounias examined the fossilized teeth of 6,500 horses representing 222 different populations of more than 70 extinct horse species, and compared the data to record of climate changes in North America over time.
Using a process called "dental mesowear analysis," they were able to look at the wear and tear on the teeth and make an estimation of what the horses ate.
"The earliest horses from (about) 55.5 million years ago had shortcrowned (brachydont) molars with poorly developed shearing crests, suggesting a frugivorous (fruit-based) diet," said the study.
Over time, grasslands became more predominant and 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," the study said.
The research suggests that larger and more evolved teeth indicate higher adaptability and a greater likelihood of survival.
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