LATE EOCENE SPATIAL VARIABILITY IN ARIDITY AND ECOSYSTEM STRUCTURE IN NORTH AMERICA BASED ON CARBON ISOTOPE RATIOS IN FOSSIL TEETH AND BONES

To investigate the spatial variability in environmental conditions in the interior of the North American continent during the late Eocene, we analyzed the carbon isotope composition of fossil bones from Toadstool Park (northwestern Nebraska), Flagstaff Rim (Wyoming), and from various fossil localities in southwestern Montana. We also analyzed the composition of fossil teeth from Toadstool Park, Flagstaff Rim, and Torrington Quarry (Wyoming). The fossil bones belong to various mammalian taxa and turtles whereas the teeth belong to the small, deer-like artiodactyl Leptomeryx and to the perissodactyl Subhyracodon (an ancestral rhino). We found the presence of significant differences in aridity and vegetation structure among these different sites during the late Eocene.

Tooth enamel is thought to be resistant to diagenetic alteration and to maintain the biological isotope signal. In contrast, bones recrystallize upon burial and fossilization and assume a stable isotope composition that reflects soil conditions. In pure C3 ecosystems, enamel δ¹³C reflects habitat preference in terms of degree of vegetation openness and/or plant water-stress. Similarly, bone δ¹³C tracks changes in vegetation and aridity. As aridity increases and the environment becomes more open, both bone and enamel δ¹³C are expected to increase.

Average (±1 SE) bone δ¹³C values from Flagstaff Rim (-6.97±0.05‰, V-PDB) and from Montana (-6.90±0.15‰) are indistinguishable. In contrast, fossil bones from Toadstool Park show a significantly lower average δ¹³C (-7.82±0.08‰). Similarly, average Leptomeryx (-9.76±0.23‰) enamel δ¹³C from Toadstool Park is significantly lower than that from Flagstaff Rim (-8.54±0.10‰) and average Subhyracodon enamel δ¹³C from Toadstool Park (-10.33±0.18‰) is significantly lower than that from Flagstaff Rim (-9.08±0.06‰) and Torrington Quarry (-8.22±0.02‰). These data indicate the presence of an east-to-west trend towards more open and xeric conditions during the late Eocene in the North American mid-continent.