Occlusal enamel complexity in beaver (Rodentia: Castoridae) genera from John Day Fossil Beds National Monument, Oregon, USA

Abstract Dental adaptations to abrasive wear include increased tooth crown height and complexity of occlusal enamel folds. These trends have been studied extensively in ungulates, and some other mammal groups with hypsodont dentition. Beavers (family Castoridae) represent diverse lineages that occupied different ecological niches through time, including the fossorial beavers of the Oligocene and the semiaquatic beavers that diversified in the Miocene. As an adaptation to the high volume of abrasive grit entering their mouths from digging with their incisors, we hypothesized that the cheek teeth of fossorial beavers had increased occlusal enamel complexity compared to semiaquatic beavers. We chose John Day Fossil Beds National Monument (JDFB) in eastern and central Oregon for this study because of its extensive time range and exceptional preservation of castorid fossils. We took scaled photographs of the occlusal surface of 43 beaver specimens (N = 84 teeth) from six genera ( Capacikala MacDonald, 1963; Castor Linnaeus, 1758; Dipoides Schlosser, 1902; Microtheriomys Korth and Samuels, 2015; Monosaulax Stirton, 1935; and Palaeocastor Leidy, 1869) from previously collected JDFB specimens. Each tooth was measured using ImageJ to calculate occlusal enamel index (OEI). We performed an analysis of variance on linear models using residual randomization permutation procedure to test the dependence of enamel complexity on taxonomy, dentition, time, geology, and ecology. Results indicate that genus, epoch, ecology, geologic formation, and geologic formations with combined units of the John Day Formation were significant, however, post-hoc pairwise comparisons resulted in no differences between groups. This suggests that enamel complexity in JDFB beavers does not significantly differ between fossorial and semiaquatic lifestyles.