Late Oligocene ambient temperatures reconstructed by stable isotope analysis of terrestrial and aquatic vertebrate fossils of Enspel, Germany

During the Oligocene, waxing and waning ice sheets on Antarctica caused global sea level fluctuations, as well as climatic and palaeoenvironmental changes that affected faunal evolution in Europe. The 24.7 Ma upper Oligocene lacustrine sediments of Enspel in western Germany have yielded a fossil vertebrate fauna of the Mammal Palaeogene (MP) reference level 28. Here, we present carbon and oxygen isotope analyses of these terrestrial and aquatic vertebrate fossils, enabling us to reconstruct the isotopic composition of the ingested food and water sources and to gain insights into the palaeoclimate and palaeoenvironment. The crocodile Diplocynodon has enamel delta C-13 values above the range expected for a carnivore feeding on terrestrial animals from a C-3 plant ecosystem, probably reflecting a diet composed mainly of C-13-enriched aquatic vertebrates from the Enspel Lake. Enamel delta C-13 values as low as -14 aEuro degrees indicate that the large ungulate mammal Anthracotherium fed in a forested environment around the Enspel maar. Anthracotherium did not predominantly ingest its drinking water from the O-18-enriched lake; Diplocynodon enamel delta O-18(PO4) values represent a delta O-18(H2O) value of -2 aEuro degrees, whereas Anthracotherium enamel gives a delta O-18(H2O) value of -6.9 +/- 0.3 aEuro degrees. This is inconsistent with a semi-aquatic mode of life for Anthracotherium, but rather suggests that it drank meteoric water, presumably reflecting the late Oligocene precipitation, thus giving a mean annual air temperature (MAT) estimate of 15.0 +/- 1.3 A degrees C. This MAT is in good agreement with previous MAT reconstructions based on plant fossils from Enspel and Anthracotherium teeth from slightly younger MP29 fossil sites in Switzerland. We suggest that a MAT of around 15 A +/- 2 A degrees C prevailed in central Europe during the late Oligocene during MP28 and 29.