Dietary change and horse evolution in North America

Paleontologists have long believed that evolutionary changes in horse teeth were caused by natural selection due to changes in diet that were caused by climate change and the appearance of these habitats. The evolution of horse dentitions remains one of the major hypotheses linking the evolution of animals to climate change. To test the hypothesis that dental evolution in horses was driven by changing diets, we investigated the wear patterns on the molars of fossil horses throughout their entire history, by recording the shape of worn molar cusps in than 6,500 fossil horses representing 222 different populations representing over 70 extinct horse species. Different diets cause the teeth of horses and other herbivores to wear differently, making it possible to reconstruct diets in extinct species by examining patterns of wear on fossil teeth. This methodology, known as dental mesowear analysis, was first developed by Nikos Solounias and Mikael Fortelius to interpret the diets of extinct species. Measuring the mesowear (sharpness of worn cusps on fossil teeth) is a way of interpreting the abrasive nature of the diets of extinct horses.

According to the dental wear patterns, we were able to verify that horse diets did indeed change along with changes in vegetation through the 55 Ma sequence.

Trend in the dietary evolution of horses in the context of climatic changes and the phylogeny of Equidae - AAAS

Grasslands began to spread in North America in the early part of the Miocene because abundant grass phytoliths are found in ancient soils from that time. We observed a shift in the mesowear patterns of horses at the same time and low abrasion diets temporarily disappeared between 22–18 Ma, suggesting that horses were pioneering these new grasslands, and adopted abrasive grassy diets. We also found that evolutionary changes in tooth anatomy lag behind the dietary changes by a million years or more. For instance, about 18 million years ago, horses with taller and more complex molars begin to appear, several million years after diets began to shift. The lag time in the evolution of horse teeth in comparison to dietary changes is critical in supporting the classic hypothesis that horse teeth evolved due to natural selection caused by changing diets. The observation that dental changes follow dietary changes is consistent with evolution due to adaptation.

Abrasion

The study revealed that most species of horses showed tremendous variation in their diets, with many populations having relatively non-abrasive diets. This discovery suggests that natural selection for dental change may have been weak or non-existent much of the time, but with punctuated intervals of intense natural selection caused by extreme dietary abrasion. The mesowear data also show that low abrasion diets began to return, after a temporary absence, about 17 Ma, and that diets of horses greatly diversified after this time, the same interval of time during which the number of horses species reached their maximum. Many horses with archaic dentitions began to gradually disappear in the late Miocene along with evidence of low-abrasion diets and by 2 Ma not only most horses had gone extinct, but the mesowear indicates that dietary diversity of this group had been greatly diminished. By 2 Ma, the only remaining horses in North America exclusively utilized high-abrasion grazing diets, like the living horses or zebras. This study shows that not only has the number of horse species been greatly reduced in the last few million years, but the diets of horses have been narrowly restricted as well. The mesowear data indicate that the living horses are anything but typical examples of the dietary ecology of this group of mammals.

For further information

Article "Dietary change and evolution of horses in North America" Mihlbachler; M.C; Rivals, F. et al. Science, 331:1178-1181, March 2011

Contact

Florent Rivals

florent.rivals@icrea.es