Revisiting the Messel palynoflora using a combined LM and SEM approach

The early to middle Eocene maar lake of Messel is a world-renowned fossil locality. Its oil shale deposits are well-known for their exceptional preservation of vertebrates (e.g., crocodiles, early horses) and invertebrates, especially insects. The makro- and microfossil remains of plants also contributed to providing a more holistic snapshot of this Eocene ecosystem. So far, the palynoflora has mostly been investigated using only conventional LM, and the classic taxonomic qualitative/illustrative work on the palynoflora dates back to the late 1980ies. Here we report on the first results from an ongoing study using combined LM and SEM, and in some cases even TEM, to re-investigate the Messel palynoflora qualitatively. The main goals of our study are to conclude (I) if some of the spores/pollen can be systematically placed with more certainty, (II) if additional spore/pollen types can be discovered, (III) to compare our combined method of investigation with the classic LM-based counting method, and finally (IV) to conclude about the composition and species richness of the palynoflora and how it correlates with the macro- and mesofloras. To accomplish this, we first processed a single sedimentary rock sample without fine-mesh sieving and then extracted and investigated every pollen type encountered using the single-grain method. So far, this sample has produced 30 spore types, 5 gymnosperm pollen types, and about 185 different angiosperm pollen types. The previous LM-based qualitative work identified 173 palynomorphs in an accumulative account from numerous rock samples up-trough the oil shale section. Our study, on a single sample from the lowest part of the oil shale revealed every single pollen type (except for Milfordia (Restionaceae) and Pityosporites microalatus (Cathaya, Pinaceae)) discovered in the accumulative approach using the classic LM counting method. Also, in addition to the c. 140 angiosperm pollen types previously recorded, we can now add at least 45 new pollen types for this locality. Our study shows that when taxonomic resolution and diversity are key, a combined LM and SEM investigation produces higher diversity than relying only on the conventional LM counting method when single samples are compared. In addition, a well-preserved single sample extensively studied using the combined approach is likely to provide the same or even higher number of taxa when compared to conventional LM counting methods analyzing an entire stratigraphic section. The qualitative combined approach will provide a more reliable presence/absence of taxa in the accumulation site hinterland. For quantitative analysis and subtle changes in the surrounding environments conventional counting methods are more appropriate because of the universal availability of light microscopes and difficulties with SEM-counting.