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The evolution of North American horses

Mesowear “ruler” developed for scoring dental mesowear on fossil equids, consisting of seven Equus cusp apices cast onto an epoxy surface. The “ruler” is used by matching the cusp apex of a fossil specimen to one of the seven reference cusps and assigning it a score of 0–6.
[Image courtesy of Matthew Mihlbachler]

Fossil horses are often referred to as classic examples of evolution. Over millions of years, they increased in body size, reduced the number of toes on their hooves and even grew bigger teeth. Now, researchers have collected vast amounts of data on that last detail—horses' teeth—from all over North America for the past 55.5 million years, when horses first appeared on the planet.

Matthew Mihlbachler and a group of researchers looked at something called mesowear in fossil horses, which describes the size and sharpness of the horses' molars, or the teeth in the back of their mouths. Molars are known to wear down from grinding against food and other teeth—and these researchers show that the changes seen in horses' teeth over millions of years are caused by changes in the animals' diets that were brought on by climate change.

Matthew Mihlbachler recording tooth cusp sharpness in a fragment of a 14 million-year-old fossil horse skull at the American Museum of Natural History.
[Image courtesy of Matthew Mihlbachler]

As an example, a horse's molars might become worn if they are suddenly forced to change their diet from fruit in the trees to grass on the ground. And in general, horses' molars grew bigger over millions of years on Earth.

The new report by Mihlbachler and colleagues shows that the increased mesowear on the horses' teeth is associated with the growth of larger molars over evolutionary time. So, it seems that the larger molars in horses' mouths were a consequence of the changing climate and their changing diets.

The researchers were able to reconstruct the entire history of what horses ate since their appearance on the planet 55.5 million years ago. But, they say most ancient populations of horses display very different amounts of mesowear, suggesting that the evolutionary pressure to grow larger molars was weak most of the time. For the most part, it seems that horses must have had it pretty easy, with food readily available most of the time.

Still, Mihlbachler and his colleagues do identify certain points throughout history where the pressure to eat grass must have been very high. One of these occurred during the early Miocene epoch, approximately 23 million years ago, just before the appearance of Equinae—the subfamily of horses that really brought those large molars into style.

Their report makes it clear that the diets of today's horses are anything but typical examples of what horses having been eating for the past 55 million years.