What We Can Learn from Horses' Evolving Tooth Enamel

What We Can Learn from Horses' Evolving Tooth Enamel

This is a Hipparionini horse from 16-12 million years ago from Nebraska.

Photo: Courtesy Nicholas A. Famoso

The evolution of the horse, from prehistoric times to the modern-day show ring, reflects a changing environment and increasing human intervention. Scientists can see this in horses' bones, hooves, DNA, and even teeth.

The complexity of enamel structure in horses’ teeth tells an interesting story about how horses evolved and, by studying equine teeth from different eras, researchers can see how the animals adapted to different grazing lands, as well as to domestication, said Nicholas A. Famoso, PhD candidate in the Department of Geological Sciences at the University of Oregon in Eugene.

“Investigating the evolution of horse enamel in North America has shown that horses had been increasing the amount of enamel in their teeth to combat the increasing amounts of grit in their diets,” Famoso said.

That “grit” comes from dirt and dust associated with pasture and grassland forage, he said. It’s like us eating fresh lettuce that hasn’t been washed well enough, and we find ourselves crunching on something gritty and sand-like.

“Chewing on grit is like chewing on sand paper,” Famoso said. Fortunately, enamel helps manage the grit—and the more complex the enamel structure, the better it deals with the grit.

“Enamel is the hardest substance in the digestive system," he explained. "Modern horses both have very tall teeth and somewhat complex enamel patterns, which enable them to eat very abrasive materials throughout their lifetimes.”

This is a Hipparionini horse from 16-12 million years ago from Nebraska.

Photo: Courtesy Nicholas A. Famoso

Humans, on the other hand, do not. “Humans have low crowned teeth, and we would wear our teeth down to nothing (eating grassland forage or gritty salads) very quickly,” he added.

In their study, Famoso and his advisor, Edward Byrd Davis, PhD, assistant professor and fossil collections manager at the University of Oregon, worked with fossils from four groups of horses. All four groups—Anchitheriinae, Merychippine-grade Equinae, Hipparionini, and Equini—coexisted 16 million years ago during the middle Miocene epoch. It was during this period that global temperatures dropped sharply, changing climates and feeding environments for these horses and other animals. Three of these groups have since become extinct, and only the Equini (“equine”) remains today.

Famoso and Davis measured the enamel pattern complexity of molars and premolars from the extinct groups of horses and their modern counterparts.

“Generally, through time, horses went from living in warm, forested environments to living in cooler, open grasslands,” Famoso said. “There is a shift from primarily eating browse (young shoots, twigs, and leaves) to eating grass. Browse is less abrasive because it’s high off the ground and doesn't accumulate as much dust, but grass is more abrasive since its right next to the ground and gets covered in dust easily.”

What’s particularly interesting, Famoso said, is that there’s a distinct difference in enamel structure between modern horses and their North American ancestors. Modern horses have “relatively low enamel complexity,” which is probably related to their “Old World” environment (modern horses all descended from Europe, Asia, and Africa—collectively, the “Old World”—since the North American species—the "New World" horses—died out about 10,000 years ago).

“New World and Old World horses evolved under different selective pressures, and the different genetic pools may be responsible for the differences we observe,” he said.

This is an Equini horse from 2 million to 10 thousand years ago from Nebraska

Photo: Courtesy Nicholas A. Famoso

Does that mean our modern horses’ teeth are poorly adapted to modern diets? “I wouldn't go so far as to say they’re poorly adapted,” Famoso said. “Feral horses in North America are possibly at a slight disadvantage compared to their ancestors. Domesticated horses that are out to pasture may also be at a slight disadvantage, but it may depend on breed.

“This may mean that domestic horses in North America will wear through their teeth a bit faster than they would have in the Old World,” he added. “It might also lead to unusual wear patterns which might result in your needing to float your horses’ teeth more often.”

Still, equine teeth are continuing to adapt even today. “Evolution is happening right in front of us,” Famoso said.

The study, "Occlusal Enamel Complexity in Middle Miocene to Holocene Equids (Equidae: Perissodactyla) of North America," was published in the open-access journal PLoS One

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at a competition stable east of Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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