AN OLIGOCENE FOSSIL TORTOISE AND THE CHALLENGE OF DETERMINING SEX

A well-preserved fossil tortoise shell (field number: SD-1-8-12) was collected from Oligocene floodplain strata in Buffalo Gap National Grasslands, South Dakota. To identify taxon and gender, several qualitative and quantitative methods were used. A CT scan was used to expose potential elements preserved inside the matrix of the shell. The scan revealed incomplete, but proximal appendicular skeletal elements. Complete shells of Oligocene tortoises are common, but rarely contain complete limb and skull material. In extant Gopherus agassizii (a close living relative), adult male tortoises contain a concavity on the plastron posterior, and well-pronounced gular projections. These characters are not always apparent in fossil specimens due to shell deformation during fossilization. Thus, museum specimens of extant G. agassizii from the University of New Mexico were evaluated for quantitative sexual dimorphic characters. Nuchal measurements show both males and females plot along a moderate linear trend, but males have larger values. Two-sample t-tests of the nuchal length and width both revealed that males are significantly greater (p-values = 0.000) than females. Sex determination becomes problematic with smaller shells of G. agassizii and may be due to those individuals not reaching sexual maturity. In contrast, two-sample t-tests conducted on fossil tortoises showed no significant distinction (p-values for length = 0.678 and width = 0.310) between male and female specimens. Likewise, nuchal length and width ratios for both G. agassizii and fossil specimens yields p-values of 0.250 and 0.508, respectively, and hence show no significant difference between male and females. Determining sex of fossil tortoises using elements of the shell requires careful examination of observable characters as well as quantified data. Based on qualitative and quantitative data, we propose that SD-1-8-12 is female. The plastron posterior of SD-1-8-12 contains a pre-fossilization deformity obscuring the presence of a concavity, and gular elements are not well-pronounced.