THE INFLUENCE OF BITE FORCE ON THE FORMATION OF DENTAL MICROWEAR IN XENARTHRANS (MAMMALIA)

Xenarthrans are a group of placental mammals (including tree sloths, armadillos, ground sloths, and glyptodonts) that lack enamel on their adult teeth, having instead an outer layer of softer orthodentine. Prior analyses of microscopic scars (termed “dental microwear”) on the chewing surface of xenarthran teeth reveal a correlation between orthodentine microwear patterns and feeding ecology, thereby providing a proxy for paleodiet in extinct xenarthrans. However, the specific formative mechanism of microwear features (e.g. scratches, pits) on xenarthran orthodentine remains poorly understood. It stands to reason that bite force (generated by mandibular closure during mastication) should influence the formation of microwear patterns, but no studies have specifically tested this hypothesis. We attempt to fill this gap in our knowledge by investigating the potential effects of variable bite force on microwear formation in tree sloths. Relative ratios of bite force were estimated from 16 skulls of Bradypus (three-toed sloth) and Choloepus (two-toed sloth) by applying a geometric model for calculating input forces from masticatory muscles (e.g., temporalis and masseter) relative to lever arm moments of the mandible. For each skull, microwear was examined on epoxy resin casts of the upper right tooth row. Using low-magnification (35×) light microscopy, we analyzed five microwear variables (i.e., number of scratches, presence of hypercoarse scratches, gouges, large pits, and cross-scratches) on four tooth positions (M1–M4) from each skull. ANOVA tests were applied to compare each microwear variable with the estimated bite force at each tooth position per taxon. While there is some positive correlation of increasing microwear variables and strength of force as one moves posteriorly, the pattern is not consistent across the entire tooth row for either taxon. The lack of a significant correlation between microwear variables and bite force values suggests that tooth scars in sloths are not being generated by pure orthal closure of the mandible, but rather are more influenced from other jaw movements. Further analyses that incorporate all masticatory muscles to create a more realistic and three-dimensional assessment of the chewing cycle should help to clarify how microwear patterns are generated in theses taxa.