Plant diversity at the final stage of the Miocene Climate Optimum – Revision of the flora of Leoben, Styria, Austria

The floras of Leoben and Parschlug (Styria, Austria) derive from nearby basins along the Mur/ Mürz fault system, representing the drowning stages of the mires in these basins. They are regarded as being similar in age. Based on its revision, the flora of Parschlug is interpreted to reflect the vegetation in the close lake surroundings and the hinterland of the respective basin. Subhumid sclerophyllous forest is concluded to be the most likely major vegetation type in the hinterland, which developed under seasonally dry climate. These results raised the question of a potential signal provided by the Leoben flora. A comparison of both floras, however, turned out to be impossible based solely on the monographical studies by Ettingshausen (1869a, 1888) and would require a comprehensive taxonomic revision, which is presented here. The Leoben flora is dated to the Middle Miocene (Langhian/Badenian, 14.9/ ±0.7 Ma). It comprises mainly leaves, but also remains of flowers, inflorescences and fructifications, mostly preserved as strongly compressed impressions. The present revision reduces the number of taxa from 382 (in Ettingshausen, 1888, excluding fungi) to ~175 or to ~150 when different organs of probably one biological fossil-species are fused. Although the taxonomic resolution is limited especially for entire-margined taxa due to the lack of cuticles, the floristic diversity is high. Spore-producing plants are generally rare. Conifers are diverse although their remains are not abundant except for Glyptostrobus and Pinus. Angiosperms include more than 30 families of which Betulaceae, Fagaceae, Lauraceae and Sapindaceae are most diverse. Two fossil-species are described for the first time and 8 new combinations of angiosperms are introduced. Phytosociologically the flora represents plants of a floating-leaf plant zone, ferns, reeds and sedges from the lake margin and of swampy habitats, as well as woody taxa from swampy forests, along streams and in the closer surroundings of the lake segueing into mesophytic hinterland forests. The Integrated Plant Record (IPR) vegetation analysis is applied to assess the most likely major vegetation type (zonal vegetation), and the Similarity Approach (application of Drudges) predicts the most likely modern vegetation proxies for the Leoben assemblage. Palaeoclimate proxies are assessed by the Climate Leaf Analysis Multivariate Program (CLAMP), and the Leoben flora is compared to floras in the Paratethys region and surroundings that are of similar age, namely Oberdorf, Parschlug (Styria), Mecsek Mts. (Hungary) and the Cypris Formation (northern Bohemia).