The Occurrence of the Fern Acrostichum in Oligocene Volcanic Strata of the Northwestern Ethiopian Plateau

Oligocene (28–27 Ma) leaf fragments displaying morphological affinities with the fern Acrostichum are described from volcanic and fluvio-lacustrine strata near Chilga, west-southwest of Gondar on the northwestern Ethiopian Plateau. The fossils consist of elongate pinnae impressions and compressions, with secondary veins that arise from a midvein, anastomose, and create a characteristic reticulate venation without included free veinlets. Modern Acrostichum has a pantropical distribution associated with mangrove vegetation, but it occasionally occurs in freshwater settings. The Paleogene fossil record shows that Acrostichum usually is preserved in freshwater environments, as it is at Chilga, but elsewhere in Africa it is associated with mangrove taxa. At Chilga, Acrostichum occurs in a waterlogged terrain, as indicated by independent environmental assessment based on paleosol analyses. This fern occupied a greater geographic range than it does today in Africa, having occurred in the past on the Ethiopian Plateau in addition to the coastal areas at tropical latitudes.