PALEOENVIRONMENT AND HABITAT OF LATE MIOCENE MAMMALS FROM HöWENEGG, SW GERMANY INFERRED FROM O, C AND SR ISOTOPE COMPOSITIONS OF FOSSIL TEETH

At Höwenegg, the northern-most volcano of the Hegau volcanic field in SW Germany, marly deposits of a shallow freshwater lake are intercalated between two Late Miocene hornblende tuff layers. Lake sediments of the Höwenegg Formation, radiometrically dated to ~10.3 Ma, are famous for the preservation of complete mammal skeletons and represent one of the first occurences of the high-crowned equid Hippotherium primigenium in Europe (Hipparion Datum) during the European Neogene Land Mammal Zone MN9.

Isotope analysis (C, O, and Sr) were made on well-preserved skeletal remains from large mammals, fish, turtles as well as gastropods and ostracods from the Höwenegg Beds to determine the palaeoenvironmental and palaeoclimatic conditions around Lake Höwenegg and to infer the habitat and mobility of the large mammals.

Enamel δ¹³C values (–11.5±0.9‰, n=29) indicate that all mammals (bovids, equids, rhinocerotids and one gomphothere) fed exclusively on C₃ plants. Using an emperical transfer function for modern mammals, the enamel δ¹⁸O_PO4 values (17.7±1.3‰, n=27) of the Höwenegg mammals can be used to determine a δ¹⁸O_H2O value of –6.8±1.4‰ for their drinking water. Using a modern-day δ¹⁸O_H2O-air temperature regression, a MAT of 15.1±3.7°C can be calculated, suggesting a significantly warmer climate at Höwenegg than today.

Small inter-taxon differences in δ¹³C and δ¹⁸O values might reflect some habitat and/or diet differences. However, intra-tooth isotope profiles for H. primigenium display no major seasonal changes in the diet or habitat. Mammal habitat use was determined by ⁸⁷Sr/⁸⁶Sr analysis of their enamel and wood from extant trees growing on the different geological units around the Höwenegg site, as well as representative rock samples to constrain the bioavailable ⁸⁷Sr/⁸⁶Sr potentially ingested by the animals.

Most large mammal teeth have enamel ⁸⁷Sr/⁸⁶Sr (0.7058-0.7068) similar to those of the tuff and the Höwenegg Beds, but lower than the Upper Jurassic limestone and Miocene Jura Nagelfluh (0.7070-0.7092) that outcrop around the Höwenegg locality today. This may indicate predominant feeding of the Höwenegg mammals on vegetation growing on the volcanic tuffs. Because horses and rhinoceros have significant homerange-sizes, this may imply a larger extension of the volcanic tuff cover during Late Miocene times.