The Rauenberg fossil Lagerstätte (Baden-Württemberg, Germany): A window into early Oligocene marine and coastal ecosystems of Central Europe

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Highlights

  • A review of the flora/fauna of an early Oligocene Rhine Graben site is provided.

  • 69% of taxa from the locality are marine, and 31% are terrestrial.

  • Deposition occurred under moderately shallow, low-energy, warm water conditions.

  • Warm, frost-free conditions led to the formation of a coastal broad-leafed forest.

  • Marine invertebrate and vertebrate faunas show distinct biogeographical patterns.

Abstract

The Oligocene was a period of profound climatic and biotic changes, coinciding with a shift from a mostly ice-free warmhouse world at the Eocene-Oligocene boundary to a globally cooler, more seasonal climate. The Rauenberg locality (Baden-Württemberg, Germany) is one of the most significant early Oligocene fossil assemblages in Europe, containing both marine and terrestrial elements of fauna and flora. Preservation is often superb and comprises complete and articulated skeletons with soft tissue preservation. The diverse assemblage provides critical insights into the paleoenvironment of the Upper Rhine Valley. We reassess diversity at the locality, resulting in a list of 302 taxa found at the site, 207 of which are marine (52% of taxa represented by macrofossils). The rest are interpreted as originating from the coastal environment. Molluscan, echinoderm, and plant macrofossil diversity are assessed here for the first time. Based on these new results, we interpret Rauenberg as representing a fully marine assemblage deposited in a moderately shallow, low-energy tropical-subtropical environment. Productivity was high, and seafloor anoxia was intermittently developed. There is no evidence for long-term brackish influence or mangrove swamps, and no direct evidence for the development of seagrass meadows. On land, warm, frost-free conditions permitted the development of prevailingly evergreen sclerophyllous broad-leaved forests along with pine and palm-rich coastal forests on sandy soils. The marine invertebrate fauna shows more northerly affinities, whereas the vertebrate fauna is distinctly Paratethyan.

Introduction

The Oligocene was a period of profound climatic changes. Its beginning, the Eocene-Oligocene boundary, marked the transition from a mostly ice-free warmhouse world to an interval of globally cooler climate and Antarctic ice-sheet formation (Coxall et al., 2005, Miller et al., 1998, Pusz et al., 2011, Zachos et al., 2008). Oligocene climate at middle and higher latitudes particularly differed from conditions during the Eocene by increasing seasonality, i.e. the development of cool winter seasons (Roth-Nebelsick et al., 2014). Fossil remains reflect this trend. Oligocene fossil plant assemblages, for example, show increasing invasion of arctotertiary floristic elements (Kvaček and Walther, 2001), and a turnover occurred within the terrestrial vertebrate community (“Grande Coupure”) (Delfino et al., 2003, Stehlin, 1910).

The clay pit “Unterfeld” at Rauenberg (Baden-Württemberg, Germany) contains one of the most significant early Oligocene fossil assemblages in Europe, and is an important locality for reconstructing early Oligocene floral and faunal diversity. Although fossils, especially those of fishes, have been known for a long time, scientific excavations at the site were intensified after the discovery of the first Old World fossils of modern-type hummingbirds (Mayr, 2004a). Preservation is often superb and comprises complete and articulated skeletons with soft tissues preserved. The Rauenberg site contains both marine and terrestrial elements of the fauna and flora. The diverse assemblage provides critical insights into the paleoenvironment of the Upper Rhine Valley and is of particular significance, because it preserves marine and terrestrial organisms across a broad taxonomic spectrum. As such, the Rauenberg site adds to an understanding of early Oligocene ecosystems in Central Europe. Located at a slightly lower latitude than its current location (paleolatitude 43.74°N, 12.81°E, Paleobiology Database rotation file: Wright et al. (2013); current: 49.27°N, 8.67°E), the locality lies in a region potentially more heavily influenced by sea level dynamics and tectonic evolution than latitudinally-driven climatic zonation. However, this is controversial and remains to be rigorously examined in a cohesive framework. In the past, independent studies have attempted to reconstruct the paleoenvironment and sedimentation regime of this locality using various techniques (e.g., micro- and macropaleontology, stratigraphy and geochemistry), resulting in inconclusive and partially contradictory interpretations. Here, we present new data and synthesize existing results to generate a more complete picture of a marine and coastal paleoenvironment in the early Oligocene of Germany. We provide a summary of the taxonomic diversity at the Rauenberg locality based on reexamination of previously collected but as-yet undescribed material, and also reassess the published literature in order to compile an up-to-date list of taxa from the locality. Some groups, such as plankton, fishes, and birds, are already well-known from Rauenberg (see Section 5 for details); others, such as plants and macroinvertebrates, have never been studied in detail. The synthesis of information from these different groups provides a wealth of new information for paleoenvironmental reconstruction.

Section snippets

Geology

The geology of the Paleogene clay pits on the eastern edge of the Upper Rhine Graben, mined since the beginning of the 19th century, has been studied by Wagner-Klett (1919), Weiler, 1931, Weiler, 1966, Sittler (1967), Doebl (1976), Schweizer (1982), Weiss (1988), Trunkó and Munk (1998), and Grimm et al. (2002).

Biostratigraphy

Among the calcareous nannoplankton, the occurrence of Transversopontis pygmaea indicates an Oligocene age between NP 23 and NP 24 (after Köthe, 1986). The section comprising the Hochberg Member is correlated to NP 23 (after Martini, 1971; modified from Müller, 1978): from the last occurrence of Reticulofenestra umbilica to the first occurrence of Cyclicargolithus abisectus and/or Helicosphaera recta. Consistent with the calcareous nannoplankton, dinoflagellate cysts in the sediments can be

Materials and methods

Macrofossils occur mainly as scattered remains in the sediment at the Rauenberg locality. Consequently, many years of intensive excavations, mainly by private collectors but more recently by the Staatliches Museum für Naturkunde Karlsruhe (SMNK), were necessary to assemble the collections at the Hessisches Landesmuseum Darmstadt (HLMD), Institut für Geowissenschaften Universität Heidelberg (GEOW), SMNK, and the Staatliches Museum für Naturkunde Stuttgart (SMNS) on which this study is based.

Systematic overview

For a complete list of taxa recovered, refer to Appendix A. The following sections are intended as a brief overview.

Biotic interactions and soft-tissue preservation

There are several bivalves in the Rauenberg material (< 10% of the total sample) bearing boreholes attributable to the ichnospecies Sedilichnus paraboloides (Fig. 4B–C). Recent representatives of both Naticidae and Muricidae are known to create holes comparable to S. paraboloides (Zonneveld and Gingras, 2014), suggesting that gastropods from these families are responsible for these traces. There are no traces of predation on Rauenberg echinoderms, in spite of the presence of Cassidae

Conclusions

Although the Rupelian assemblage from Rauenberg is composed predominantly of marine taxa, 95 of 302 taxa currently identified from the locality are terrestrial (31% total, or 48% of taxa represented by macrofossils). Based on new results, we interpret Rauenberg as a fully marine locality, deposited in a moderately shallow, low-energy tropical-subtropical environment. Productivity was high, and seafloor anoxia was intermittently developed. There is no evidence for long-term brackish influence or

Acknowledgements

Thanks to the Klaus Tschira Stiftung and the municipality of Rauenberg, who financially and logistically supported our work at the Unterfeld clay pit. S. Staudt, K.-L. Metzger, and many unnamed student assistants helped with excavations, and H. and A. Oechsler collected a significant amount of material. Thanks to M. Keller and C. Knopf for donation of fossils, K. Weiß for the preparation of some bird specimens, W. Munk (SMNK) for information about the insect material preserved at SMNK, M.C.

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