Dental wear pattern analysis of Stephanomys donnezani (Muridae, Rodentia, Mammalia) from the Pliocene of Layna (Soria)

Abstract

The Pliocene site of Layna is a karst filling in the Iberian System (López Martínez, 1989) placed on the northern edge of the Tagus Basin. Its outstanding abundance of mandibles and maxillas makes Layna a good test-case locality to apply statistical methods in studies of fossil assemblages, minimizing the possible biases present in poorer sites. The objective of this work is to analyze the wearing of the different molar elements, taking for the first time an approach that can study this issue in teeth from the same individual. We chose Stephanomys donnezani (Depéret, 1890) because it is one of the most abundant taxa in Layna. We followed the method proposed by Freudenthal et al. (2002) which described the Wear Index (WI) as the ratio between exposed dentin surface and the total occlusal surface of the molar. We took photographs and digitized the occlusal surface of 167 molars, calculating the area of the 2D projection of the occlusal surface and the exposed dentine in pixels using Adobe Photoshop CS4 software. Molar wearing was compared among the different elements by means of Spearman correlation of percentage histograms as well as by Mann-Whitney U tests. Linear regression analyses were used to evaluate differences in wear among elements of the same dental row. Molars were considered to have same wear rate when slopes of those regressions ranged between 0.9 and 1.2. The results of the Spearman correlations and Mann-Whitney U tests indicate that most of the elements showed similar wearing levels. Nevertheless, when studying teeth from the same individual, althoughthere were no significative differences between m1 and m2 (slope=1.006,r2=0.744,p<0.001), the m3 had lower WI values than both of them (slope=0.726,r2=0.584,p<0.001 when related to m1; slope=0.621,r2=0.761,p<0.001 when related to m2). This could be explained by the fact that m3 arises later than m1 and m2. We concluded that young individuals probably take some solid food, wearing their teeth during the last phases of the breastfeeding period and before the m3 arises. Regarding the upper molars, the three elements wore in the same way. However, the number of maxillas is low and this result has to be taken carefully.