Meeting Abstract
Post-natal growth in vertebrates reflects repositioning on the trophic scale, whether through changes in size or diet. Dietary shifts through growth are reflected in changes in cranial morphology accommodating differing proportions of musculature, reconstructed via allometry. Although allometric growth is readily measurable in extant taxa, fewer extinct taxa are represented by sample sizes that allow for study of allometric shifts. Furthermore, most fossils are either fragmentary or deformed during the fossilization process, making comparable measurements across specimens difficult. Here, we describe allometry in the cranium of the large-bodied herbivorous stem mammal, Exaeretodon argentinus. Nearly all specimens were deformed, and not typically useful in studies of allometry. To combat this, we simulated distorted and normal data, under a known allometric coefficient, and tested model fit of a simple linear regression and a mixed effects model, which can incorporate additional sources of variation. We simulated differing sample sizes, and found that across all sample sizes, a mixed effects model is better able to predict the allometric coefficient, given deformed data. Under a mixed effects model, we find that through growth E. argentinus reduces snout length, widens the back of the skull, and expands the masseter attachment site while reducing the temporalis attachment site. These shifts suggest chewing-dominated feeding in adults, which indicates that smaller individuals had a more crushing feeding style, consistent with a lower proportion of plant material in the diet and a shift from omnivory to herbivory during ontogeny.
