CHANGES IN DIET THROUGH TIME IN THE MESOGAULINAE (MAMMALIA: RODENTIA: MYLAGAULIDAE) IN THE CONTEXT OF THE SPREAD OF GRASSLANDS: MICROWEAR RESULTS USING TEXTURE ANALYSIS AT HIGH MAGNIFICATION

The Mylagaulidae, a family of fossorial rodents from the Miocene of North America that diversified in conjunction with the spread of grasslands, have been regarded as having an abrasive diet of grasses. This diet is hinted at by their hypsodont teeth and simplified dentition with an enlarged fourth premolar triggering the loss of the first molar throughout ontogeny. However, no formal investigation of this diet has been undertaken.

Paleontologists have deciphered the feeding habits of several diverse fossil species using scanning electron microscopy. This technique allows the observation, characterization, and quantifying of microwear features, such as scratches and pits on the tooth enamel, consequences of the consumption of food by the animal. A variation on this technique is here used to assess the diversity of diets among basal mesogaulines, a group of highly specialized burrowers. The method used relies on the texture analysis of the imaged 4^th premolar (the largest and most important tooth in food processing) rather than the interpretation of high magnification images. This novel approach is much more quantitative than the traditional one and provides quite precise values for numerous microwear parameters otherwise unavailable. Several profiles of the p4 in different orientations are used to assess parameters such as average roughness and maximum height. These profiles were randomly selected in order to account for variations in microwear signature within the tooth at high magnification. This technique allows different diets within the fossil group to be discriminated.

Rather than a homogeneous diet of grasses, it appears that the diets of mesogaulines exhibit some heterogeneity with various degrees of grazing. Some species were found to have a diet quite close to that of Aplodontia rufa, the closest living relative of mylagaulids, that feeds on a great variety of plants including ferns, toxic plans, alder leaves and conifers, whereas others such as Alphagaulus douglassi exhibit a microwear signature indicative of more grazing. Preliminary results on changes in diet with the evolution of the mesogaulines seem to corroborate the hypothesis of an adaptation of these rodents to a more open environment with grasslands. Future work using the technique presented here should allow further diet studies of fossils.