Diversity and ecological significance of fern dominated horizons from the Late Oligocene on the northwestern Ethiopian Plateau, Chilga.

GARCÍA MASSINI, J.L., JACOBS, B.F.

Chico, California, USA

Congreso; 100th Botanical Society of America meeting; 2006

Abstract This paper describes the fern flora diversity present in the Chilga region (Late Oligocene), on the northwestern Ethiopian Plateau. To date, assessment of the paleoecology of plant macrofossils, palynomorphs, vertebrates, sediments, and qualitative interpretation of paleosols indicate that the Chilga strata were deposited in a generally fluvio-lacustrine environment on a fairly flat-lying plateau in a tropical humid climate. Systematic collection from three closely spaced stratigraphic sections through a major volcanic ash depositional sequence yielded an aquatic and terrestrial fern flora that consists of vegetative and fertile frond and leaflet impressions and compressions assignable to five different taxa (Salvinnia, Marselia, Acrostichum, and an two unidentified pinnatifid fern), consistent with a moist tropical climate. Sedimentology, lithology, and paleosol type analyses of the described sections indicate rapid sedimentation of airfall and reworked ash, which alternates laterally and vertically with the occurrence of lignites and development of paleosols corresponding to the depositional processes of a migrating floodplain river. In some layers these ferns occur either singly or in combination as the dominant vegetation, typical of pioneer vegetation in primary successional stages. Tropical inland areas preserving plant assemblages from the Paleogene in Africa are rare, and the degree of diversity, quality of preservation, and abundance of fern-bearing localities found at Chilga are exceptional, providing the first view of dynamic paleoecology in response to volcanic disturbance in the tropical African highlands.