EARLY MIOCENE PALEOENVIRONMENTS OF THE HIWEGI FORMATION ON RUSINGA ISLAND (EQUATORIAL AFRICA, LAKE VICTORIA, KENYA) AND THEIR IMPLICATIONS FOR HOMINOID EVOLUTION

The early Miocene evolution and diversification of cercopithecoids and hominoids is well documented in East Africa. Fossils from Rusinga Island, Kenya are a key paleobiological record of this, preserving the remains of at least two ape genera and three other catarrhines, in addition to more than 100 species of vertebrates and abundant plant and invertebrate fossils. The majority of the fossils collected from Rusinga Island have been collected from the ~50-80 m thick Hiwegi Formation, which is sub-divided (in ascending stratigraphic order) into the Kaswanga Point, Grit, Fossil Bed, and Kibanga Members. Over the past decade we have conducted detailed geological and paleontological studies through the Hiwegi Formation, which we report here. Our analyses indicate that the Hiwegi Formation was deposited between ~18.0 and ~18.3 Ma. Within the Hiwegi Formation, there are two major vertebrate fossil-bearing intervals that sample considerably different environments. Fossils from the older fossiliferous interval, which occurs from ~18.1 - 18.2 Ma, are from the Grit Member and Fossil Bed Members. Evidence for paleosols, sedimentological characteristics of the deposits, fossil floras, and the fossil fauna all indicate that this interval samples a relatively open woodland or tropical seasonal forest biome that existed in a highly seasonal, hot and dry paleoclimate. In contrast, analyses of the paleosols, sedimentology, fossil floras and fauna of the younger interval, which was deposited ~18.0 Ma and occurs near the top of the Kibanga Member, indicate that the landscape was covered by a widespread, closed canopy tropical seasonal forest in a seasonal, warm and wet paleoclimate. These results indicate that the paleoenvironments and paleoclimates of the Hiwegi Formation changed dramatically through time and that historical fossil collections are biased, time-averaged assemblages that sample a variety of environments. Importantly, the same ape and catarrhine taxa occur in both paleoenvironments, demonstrating that early Miocene hominoids lived in both more open and more closed habitats – and likely the transition between the two. Thus, basal hominoids were biologically adaptable to changing climates and habitats, which suggests biological adaptability to environmental change is a characteristic of hominoids.