326-4 How Did Rhinoceroses Grow? A Systematic Analysis Of Growth Pattern in North American Rhinoceroses

Abstract:

Rhinoceros growth and development is not fully studied as a group. Despite the numerous fossil specimens available for research, it is not fully understood whether isometric or allometric growth takes place during their ontogeny (over the course of their lifetime) or phylogeny (over the course of their evolution). To get a good idea across the different groups across their ontogeny, I measured the limb bones of two species: the most robust of the group, Teleoceras proterum, as well as the most gracile, Menoceras arikarense. We measured diaphyseal length (minus the epiphyseal caps) and midshaft circumferences of the humerus, radius, femur, and tibia of both rhinos from several museum collections. We calculated the reduced major axis (RMA) slope for their growth, using PAST software. The slopes for most of the Menoceras limbs were slightly greater than 1.0 (slightly gracile), while most of the Teleoceras limbs had slopes less than 1.0 (slightly robust). However, these slopes are not significantly different from the isometric slope of 1.0 because the error estimates in the slope are large. This is consistent with the data from Kilbourne and Makovicky (2012), who found that even graviportal animals like the black rhinoceros (Diceros bicornis), as well as elephants and hippos, showed isometric growth in most of their limbs, despite their large body masses.

In addition, I conducted a separate analysis to see how Teleoceras limb bones evolved over time as their limbs got more and more stumpy.  As predicted, the later species of Teleoceras show negative allometry, consistent with the idea that the limbs are growing thicker faster than they are growing longer. However, since these interspecific comparisons can use only means of species samples, there are only a handful of data points, so the correlation coefficient is quite low, and these results were not significantly distinct from the isometric slope of 1.0.

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