STABLE ISOTOPE ECOLOGY OF RHINOCEROSES (MAMMALIA: FAMILY RHINOCEROTIDAE) IN THE HEMPHILLIAN LAND-MAMMAL AGE OF THE GREAT PLAINS, NORTH AMERICA

Members of the family Rhinocerotidae were one of most successful mammal groups of the Oligocene and Miocene in North America. Their extinction in the early Pliocene has been attributed to several causes associated with environmental change. This study uses stable isotopes to test whether or not the last North American rhinoceroses, Aphelops and Teleoceras, were able to adapt to the expansion of C₄ grasslands in the Great Plains during the Hemphillian land-mammal age.

Stable carbon isotopes were sampled from the tooth enamel carbonate of 38 Teleoceras and 22 Aphelops specimens, spanning the Hemphillian and Hh2 to Hh4 biochrons, respectively. Mean δ¹³C_E values of Teleoceras in Hh1 through Hh4 are -8.4±0.5‰ (1 S.D.), -9.1±1.0‰, -8.8±1.2‰, and -10.0±1.0‰, respectively. Mean δ¹³C_E values of Aphelops in Hh2 through Hh4 are -8.7±1.5‰, -10.1±2.2‰, and -10.7±0.7‰, respectively, exhibiting a wider range of values than in Teleoceras. The majority of samples for both genera plot in dry, water-stressed C₃ habitats, with the possibility of a small C₄ component, while a smaller number of individuals plot in the wetter, open canopy C₃ range. A few individuals show unequivocal C₄ signals (>-7.7‰). Aphelops and Teleoceras δ¹³C_E values exhibit no significant temporal changes (Aphelops, p>0.15; Teleoceras, p>0.05; Mann-Whitney) and cannot be statistically distinguished from each other in any biochron or any single locality in which they co-occur (p>0.09, Mann-Whitney).

Results indicate that both rhinos remained primarily C₃ feeders during C₄ expansion, with the ability to incorporate only a small amount of C₄ in their diet. In contrast, horses show greater C₄ consumption at this time. The high δ¹³C_E values in most Aphelops and Teleoceras, with many plotting in the water-stressed C₃ range, suggest that these rhinos occupied primarily dry, open habitats, such as grasslands.

Rhinoceros δ¹⁸O_E values generated for this study decrease latitudinally (δ¹⁸O_E [‰] = -1.04 [±0.07] * Latitude [°] + 67.02[±8.11], r² = 0.74, p<0.001), indicating a strong latitudinal δ¹⁸O gradient comparable to those recorded in other stable isotope proxies, probably reflecting a temperature gradient. This suggests that these large herbivores effectively tracked δ¹⁸O values in meteoric water and are potentially useful for paleoclimate reconstructions.