High taxonomic resolution SEM study of pollen and spores from the 21.73 Ma Mush flora, Ethiopia, Africa

The early Miocene Mush flora has the potential to provide exceptional insight into the paleoecology and floral biogeography of East Africa, which is otherwise deprived of fossil plant assemblages from that time. However, at present only three genera within two families, Englerodendron (Fabaceae: Detarioideae), Newtonia (Fabaceae: Mimoseae), and Tacca (Dioscoreaceae), have been described from the plant macrofossil record. The remaining c. 50 leaf morphotypes have been classified on the basis of their morphological features and require additional cuticle analyses and comparative study. Previous work on the palynoflora was based on light microscopy (LM) only and comprised 32 palynomorphs, representing 24 families – ferns (single family) and angiosperms. To assist in the identification of the macrofossils and to establish a more complete picture of the paleovegetation, we are investigating pollen and spores from three (out of six) of the same stratigraphic levels from which leaf macrofossil censuses have been conducted. The primary results from this combined LM and SEM study follows. The palynoflora is composed entirely of fern spores and angiosperm pollen with a notable absence of gymnosperm pollen. So far we have discovered 15 different types of fern spores, and an amazing number of different angiosperm pollen types. Interestingly, monocots are not diverse, and so far we have only identified two types, pollen of Sclerosperma (Arecaceae) and that of Pontederiaceae. The former a typical swamp element in tropical Central West Africa and the latter aquatic plants with a pan-(sub)tropical distribution. The dicot component is the most diverse with 85 different pollen types discovered so far. The highest diversity occurs in the Fabaceae (8 spp.) and the Sapotaceae (7 spp.). Other families, represented by a number of taxa, include Apocynaceae, Araliaceae, Euphorbiaceae, Malvaceae sensu lato, Moraceae, and Vitaceae. This rich angiosperm palynoflora reflects a diverse and complex forest vegetation surrounding the ancient lake. The forest was composed of various trees, shrubs, and woody climbers, thriving under a hot and humid climate. The high terrestrial diversity together with striking wetland and aquatic components hint at vegetation under tropical rainforest-like conditions. Future work will complete the SEM study from the censused levels and provide a reliable illustrated taxonomic list that will be used for pollen/spores counts within the Mush section. This will provide a final quantitative analysis based on the qualitative SEM work now in progress.