OCCLUSAL ENAMEL COMPLEXITY OF BEAVERS (RODENTIA: CASTOROIDEA) FROM JOHN DAY FOSSIL BEDS NATIONAL MONUMENT, OREGON

Beavers occupied different ecological niches through time, from the burrowing Palaeocastorinae beavers in the Oligocene to the semi-aquatic Castorinae and Castoroidinae beavers appearing in the Miocene. As fossorial beavers dig using their incisors, we hypothesize that their cheek teeth had higher occlusal (chewing surface) enamel complexity than semi-aquatic castorids to adapt to the high volume of abrasive grit entering their mouths from digging through substrate with their teeth. We chose John Day Fossil Beds National Monument (JODA) in eastern and central Oregon for this study because of the extensive time range through most of the Cenozoic and the exceptional preservation of paleontological material. JODA also includes both fossorial and semi-aquatic castorids. We took scaled photographs of the occlusal surface of 33 castorid specimens (n = 75 teeth) from four extinct genera (Capacikala, Dipoides, Microtheriomys and Palaeocastor) from JODA collections. Enamel length and true area on the occlusal surface of each tooth was measured using ImageJ to calculate occlusal enamel index (OEI) values. OEI was reciprocally transformed to fit normal distribution for analysis to not violate the assumptions of parametric statistical tests. We performed two multivariate analyses of variance (MANOVAs), one for upper dentition and one for lower dentition, with OEI as the dependent variable and geologic age, tooth position, geologic unit, genus, North American land mammal age, and locomotor ecology as the independent variables. Preliminary MANOVA results indicate geologic unit (p < 0.0006) was significant for enamel complexity in both upper and lower dentition and tooth placement (p = 0.03326) was significant for only lower dentition. This suggests that enamel complexity in castorids is uniform across all taxa and locomotor strategies but is inconsistent through time. Further analysis aims to investigate the influence of phylogeny on occlusal enamel complexity in this clade.