Early Oligocene plant diversity along
the Upper Rhine Graben:
The fossil flora of Rauenberg, Germany
1 | State Museum of Natural History Stuttgart, Rosenstein 1, 70191 Stuttgart, Germany |
Online publication date: 2016-12-13
Publication date: 2016-12-13
Acta Palaeobotanica 2016; 56(2): 329–440
ABSTRACT
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.