A second nearly complete skeleton of the fossil ailurid Pristinailurus bristoli was recently recovered from the Hemphillian (late Miocene) Gray Fossil Site of northeast Tennessee. It appears to be significantly larger than the previously known skeleton of P. bristoli, which was surprising as the living red panda Ailurus fulgens is thought to be generally monomorphic, including between sexes. Here the presence of significant size difference in the geometric means of cranial and postcranial measurements and canine area in the two specimens of P. bristoli is tested relative to the distribution of size differences in intersex and intrasex pairs of A. fulgens. The presence of sexual dimorphism in these dimensions is also assessed in A. fulgens using t-tests. We report the first evidence of sexual dimorphism in both upper and lower canine area in A. fulgens. We also found evidence of significantly greater body size difference in the two specimens of P. bristoli than that present in A. fulgens. This could reflect a variety of causes, including the presence of multiple species or subspecies, a greater range of ecologically driven variability, or the presence of sexual dimorphism. We argue tentatively for sexual dimorphism as the most plausible explanation for the apparent size range of P. bristoli.

The well-known symbol for conservation, the giant panda bear, and the original red panda have been forced into remote habitats due to anthropogenic disturbance, making ecological study difficult. Therefore the first known species distribution model was created to predict the most likely areas of occurrence within the known range of these elusive animals. These models were then projected onto North America and evaluated against existing breeding programs. Additionally, the close proximity of the Gray Fossil Site and the discovery of the most complete fossil red panda specimens in the world allowed ecomorphological comparisons between the modern red panda, Ailurus fulgens and the fossil red panda, Pristinailurus bristoli. Spatial data and morphometric results from this study will aid conservation work and breeding programs globally.