Paper No. 24
Presentation Time: 9:00 AM-6:30 PM
COMPARATIVE ECOLOGIES OF MIDDLE EOCENE MAMMALS INFERRED FROM STABLE ISOTOPES
Throughout the Eocene, the distribution and composition of terrestrial faunas underwent episodes of turnover as high-latitude barriers to immigration were emplaced or removed via climate or sea level change. One notable example occurred towards the end of the middle Eocene, when anthracotheres, true rhinocerotids, and amynodontid perissodactyls appeared in North America. The ecological differences between these new taxa and resident species has not been previously explored, but it is possible that the appearance and dispersal of these species into North America may have been facilitated by the ecological preferences of these groups and their capacity to disperse across high-latitude corridors. Here, we tested this hypothesis by examining the ecological preferences of similar taxa from two middle Eocene faunas (Pondaung Formation, Myanmar; Hancock Quarry, Clarno Formation, Oregon USA) through stable isotope analysis. As carbon and oxygen isotopic compositions of tooth enamel can provide insight into the diet and habitat preferences of extinct taxa, we used this information to infer the degree of ecological similarity between taxa from these two faunas. Further we used these data to estimate climatic parameters, specifically paleoprecipitation, to explore differences in ecological setting.
Tooth enamel from last molars (M3/m3) of four different families (Agriochoeridae, Brontotheriidae, Rhinocerotidae, and Amynodontidae) was powdered and prepared for stable isotope analysis of the structural carbonate component. Statistically significant differences in enamel δ13C values (Student t test, t = -6.834, p < 0.01) and enamel δ18O values (Student t test, t = 3.900, p < 0.01) between faunas were consistent with expected environmental conditions for tropical and temperate locales. Mean annual precipitation (MAP) for each site was estimated from enamel δ13C values, which showed wetter conditions at the Pondaung locale (MAP = 1192 mm) than at Hancock Quarry (MAP = 267 mm). It was inferred that all of the mammals analyzed had a diet dominated by C3 plant material, but variation in δ13C values was greater among the Pondaung mammals. No taxon specific effect was found, although rhinos and amynodonts from Hancock Quarry had slightly higher values than co-occurring brontotheres and agriochoeres.