RECONSIDERATION OF THE STRATIGRAPHY OF RUSINGA ISLAND, LAKE VICTORIA KENYA AND ITS IMPACT ON FLORAL AND FAUNAL EVOLUTIONARY INTERPRETATIONS

Nearly a century of geological and paleontological research on Rusinga Island, Lake Victoria, western Kenya, has made it a central figure in interpretations of early Neogene floral and faunal evolution in Africa. Motivated in large part by the discovery there of fossil primates, Rusinga now boasts more than 100 species of mammals preserved in sediments dating from ca. 20-17 Ma. Much of the geological and paleontological work has focused on the fossil-rich Hiwegi Formation, whereas other formations have produced notable finds but are generally less well understood stratigraphically and ecologically.

Here we report three major advances in our understanding of the oldest sediments from Rusinga. New field mapping and stratigraphic analyses of the type section of the Wayando Formation reveal that it is equivalent to the upper member of the Kiahera Formation, rather than an older, distinct stratigraphic formation. Strata that have been mapped as the Wayando Formation on neighboring Mfangano Island are instead part of an older unit informally called the Makira Series. Further, analyses of sedimentology and paleosol types from the Gumba Red Beds (fossil site R74), previously not placed within the stratigraphy of Rusinga, allow us to tentatively place R74 within the upper member of the Kiahera Formation.

At the R74 locality we report the discovery of a new infilled tree stump cast with an abundance of fossil vertebrates. This tree stump cast was found in growth position surrounded by a clay-rich paleosol, which we interpret as a Vertisol. Unlike well-known fossils from “Whitworth’s Pothole,” specimens in this hollow tree seem to have been accumulated by a carnivorous animal with a preference for a smaller size-class of prey. Remarkably, this site samples much of the variety of mammals within this size-class that have been found elsewhere on Rusinga, including very rarely discovered species. It also preserves important evidence of previously unknown taxa including a new eulipotyphlid, multiple new rodent species, the skull of a new chiropteran, and a new lorisid primate.

These results substantially advance our understanding of the stratigraphic relationships among fossils sites on Rusinga Island, and especially contribute to knowledge of the small mammalian communities present during deposition of the Kiahera Formation.