DO CHANGES IN THE TOOTH MORPHOLOGY OF MYLAGAULID RODENTS REFLECT ADAPTATIONS TO A CHANGING ENVIRONMENT?

Mylagaulids were the dominant burrowing rodent clade during the later part of the Miocene. A large number of species is known from western North America as well as China. Despite their apparent success through the end of the Hemingfordian, the Barstovian, and much of the Clarendonian North American Land Mammal ages (ca. 18 to 10 Ma), they declined during the Hemphillian and went extinct around 5 Ma. Previous research has suggested changing vegetation played a role in their demise but little ecomorphological evidence supports this hypothesis. We used X-ray microcomputed tomography to analyze the morphology of 38 mylagaulid teeth from Montana, Nevada, and Oregon spanning over 10 million years of their evolutionary history from the Hemingfordian to the Hemphillian. We quantified patterns of changes in enamel volume, thickness, and complexity. These variables have been demonstrated to reflect dietary preferences and environmental affinities in various mammalian clades. We expected an increase in volume, thickness, and complexity of tooth enamel in mylagaulids through time with the spread of an increasingly open, savannah-like environment in North America.

The six species sampled show an increase in enamel volume, even after correcting for the increase in tooth size through time. This is associated with increasing occlusal enamel complexity. Younger species have more numerous enamel lakes with more convoluted outlines. When accounting for changes in tooth size, there is no significant change in enamel thickness through time. Rare specimens of very large young mylagaulids show particularly large volumes of very intricately folded enamel. It thus appears that mylagaulids coped with an increasingly open and xeric environment, leading to tougher foods and more grit, through increasing tooth size, increasing relative enamel volume, and increasing enamel complexity. These results also suggest that mylagaulids may have reached a threshold in the volume of their teeth composed of enamel that led to their extinction.