Early Oligocene plant diversity along the Upper Rhine Graben: The fossil flora of Rauenberg, Germany
Authors: Kovar-Eder Johanna 
Published: December 2016

The macroflora of Rauenberg, Baden-Württemberg, Germany, is treated monographically. The plant-bearing sediments are marine, mainly well-bedded clay- to siltstones, the so-called Fischschiefer, which are part of the Bodenheim Formation. Based on nannoplankton they are dated to nannoplankton zone NP 23 (Rupelian, Lower Oligocene). The plant remains, mainly leaves and some fructifications, are preserved as compressions. The taxonomic assignment is based on gross morphology and cuticle characteristics. The flora yields marine algae and remains of the very diverse terrestrial flora. A total of 68 taxa, including three types of algae, one cycad, 12 conifers, and 49 dicots, among them 5 palms, are described. The following fossil species are described for the first time: Laurophyllum rauenbergense, Myrica obliquifolia, Distylium metzleri, ? Berchemia altorhenana, ? Ternstroemites maritiae, Trachelospermum kelleri, Oleinites altorhenana, O. rauenbergensis, Dicotylophyllum badense, D. oechsleri, D. vesiculaeferens, D. ziegleri, ? Viscophyllum hendriksiae, and Cladites vesiculaeferens. Dicotylophyllum vesiculaeferens and Cladites vesiculaeferens bear peculiar, complex cuticular structures presumably representing salt-secreting glands. Both taxa are interpreted to derive from one plant species of yet uncertain systematic affinity. The flora bears a high proportion of broad-leaved, presumably evergreen taxa, whereas the diversity of modern Arcto-Tertiary taxa (sensu Kvaček 1994) is rather low. Most abundant are Platanus neptuni, Daphnogene cinnamomifolia, and Tetraclinis salicornioides. On the family level, Lauraceae (10 species) and Pinaceae (8) are most diverse, followed by Arecaceae (4-5), Cupressaceae, and Myricaceae (4 species each). Surprisingly, Fagaceae are documented solely by a single leaf of Eotrigonobalanus furcinervis f. haselbachensis, and the record of Pentaphyllaceae remains ambiguous (? Ternstroemites maritiae). Sloanea olmediaefolia is recorded for the first time from western parts of Europe. Remarkable is the presence of the rare cycad Ceratozamia floersheimensis. The following possible vegetation units are suggested: zonal broadleaved sclerophyllous evergreen forests and an intrazonal coastal pine-laurel-palm association on near-coastal sandy soils, as well as gallery forests along streams. No records of swampy environments were recovered. The climate may be characterised as follows: Cfa climate in transition to Cwa (and Am or Af) climate sensu Köppen, mean annual temperature 19-24°C, mean annual precipitation 1300-1700 mm, mean temperature of the warmest month 28-29°C, mean temperature of the coldest month 8-14°C, mean precipitation of the wettest month >230 mm, mean precipitation of the driest month 18-38 mm, wettest month between May and October, driest month between November and March. The warm period was the wetter one.

The flora from Rauenberg most closely resembles that of Flörsheim (Kvaček 2004a) and shows relations to the Paratethys realm, for example the Tard Clay Formation. Relations to the floras from Saxony, Saxony-Anhalt, and North Bohemia, similar in age, are rather restricted: broad-leaved deciduous taxa are much less diverse, and the numerous presumably evergreen taxa and palms present in Rauenberg have not been recorded in the other regions, indicating a more complex vegetation differentiation than a simple north-south gradient. The high number of taxa of uncertain affinity at Rauenberg points to the need for further taxonomic studies of the flora of this time interval. Comparisons with European assemblages of the early Oligocene reveal that the vegetation diversity in Europe during this time interval is far from being well understood.