March 5, 2011

New York Cosmos

Using fossilised teeth, researchers have reconstructed the diets of extinct North American horses to show that anatomical changes in the molars of horses were an evolutionary adaptation resulting from natural selection.

A new study in Science reports that the changes to horse teeth coincide with the spread of grassland habitats across the North American continent between 18 and 22 million years ago, suggesting the adaptation was prompted by dietary changes resulting from climate change.

“Throughout their 55 million year history, we found that the diets of horses were strongly influenced by climate and vegetation changes,” said co-author Matthew Mihlbachler, from the New York College of Osteopathic Medicine. “We also found that evolutionary changes in tooth anatomy lagged behind the dietary changes by one million years or more... supporting the classical hypothesis that horse teeth evolved due to natural selection caused by changing diets.”

Linking adaptions in teeth to diets

Palaeontologists have long held that the structural changes in horse teeth signalled the rise of new grassland habitats.

However, pinpointing how these environmental shifts influenced the feeding patterns of horses was difficult because many species were extinct.

Through extensive analysis of the fossil record, Mihlbachler and his colleagues were able to link the adaptations in horse teeth – which included taller molars and more complex folds in the enamel – to dietary changes caused by climate change.

Tracking the more varied diet

The researchers also found that most species of extinct horses exhibited a high level of variation in diet, suggesting that evolutionary changes resulted from distinct periods when shifts in feeding patterns changed dramatically.

As the grasslands spread, horses began pioneering these environments and feeding on grass, which is considered more abrasive than tree leaves because it contains small sand-like granules, dirt and grit.

“This surprising discovery suggests that natural selection for dental change may have been weak or non-existent much of the time, but with episodes of intense natural selection during times when horses were forced to adopt extremely abrasive diets,” said Mihlbachler.

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