Examination of an Oligocene lacustrine ecosystem using C and N stable isotopes

The Late Oligocene (25.8 Ma) Enspel Fossillagerstätte in Westerwald, Germany, contains a comprehensive fossil ecosystem preserved with specimens retaining morphological detail and a concentration of organic material. Stable carbon and nitrogen isotope analyses were used to examine the lacustrine ecosystem preserved in one stratigraphic horizon. These data suggest the presence of several trophic levels, including primary producers (diatoms and higher plants), primary consumers (e.g., tadpoles and insects), and secondary consumers (e.g., the fish species Paleorutilus enspelensis). Terrigenous and aquatic plants were associated with the lowest δ¹³C and δ¹⁵N values (mean plant = ? 26.28‰ ± 0.45, 3.18‰ ± 1.04), primary consumers such as flies are one trophic level higher, and carnivores such as fish are yet another level higher. The δ¹⁵N values for P. enspelensis also showed enrichment in ¹⁵N with increasing body length, implying a shift in diet or feeding strategy with size. P. enspelensis and tadpole (Pelobates decheni) samples showed intraorganism fractionation between ‘muscle’ and ‘bone’ tissues. Stable carbon and nitrogen isotope data from the measurement of components (shale, leaves and seeds) common to a number of different stratigraphic horizons showed significant variation between horizons. A number of the features of the stable isotopic data are similar to those relationships seen in modern ecosystems and therefore suggest that stable isotope analyses can contribute to understanding ancient ecosystems.