SEXES OR SPECIES? CASE STUDIES IN CARNIVORAN BODY SIZE AND TAXONOMY FROM GERMANY AND IDAHO

Fossil mammal species are often identified based on differences in body size. Distinguishing species based on size is especially common in small carnivorans. However, many living carnivorans exhibit strong sexual dimorphism, suggesting that some putative species pairs in the fossil record may represent males and females of the same species. Are size differences between these pairs consistent with those seen in a single sexually dimorphic species, and can the shape of body size distributions provide insight into distinguishing species pairs from sexually dimorphic species? We addressed this question by collecting body mass data for small extant carnivorans (Mustelidae, Mephitidae, Procyonidae, Ailuridae, Viverridae, and Herpestidae) from online museum databases. We determined whether males and females differed significantly from one another in size and calculated mean mass for each sex as well as the skewness and kurtosis of their distributions. We also gathered body size data from the literature for two species pairs of small fossil carnivorans from a single site distinguished based, at least in part, on size (Palaeogale from the Miocene of Amöneburg, Germany and Trigonictis from the Pliocene of Hagerman, Idaho). Body size patterns in modern carnivorans suggest that sexual dimorphism can be due to unequally skewed distribution between sexes rather than simply differences in mean mass. This may provide a means of distinguishing sexually dimorphic species in the fossil record based on body size distribution. In Palaeogale, the differences in body size between putative species falls well within the range of differences between males and females of modern sexually dimorphic carnivorans, calling the validity of these taxa into question. However, our sample of Palaeogale specimens is too small to small to draw any meaningful conclusions about body size distributions. The two Trigonictis species found in the Glenns Ferry Formation at Hagerman Fossil Beds are each represented by much larger fossil samples, and these too differ to roughly the same degree as seen in modern sexually dimorphic carnivorans. Analysis of Idahoan Trigonictis fossils will allow us to reconstruct size distributions in these taxa to see whether they confirm the long-standing interpretation of these fossils as representing two separate species.