The paleoecology and paleogeographic contet of Lemudong'o locality 1, a late Miocene terrestrial fossil site in southern Kenya

Abstract

The Lemudong’o Formation in the Narok District of Kenya comprises a 135-m- thick series of predominantly lacustrine and lake basin margin sedimentary rocks with in terstratified primary and reworked tuffs. The formation, deposited , 6 Ma, records deposition within the second of three sequential lake basins created by tectonic and volcanic activity on the we stern margin of the southern Rift Valley of Kenya. These sedimentary paleobasins are exposed in the vicinity of the confluence of three rivers cutting steep cliffs into rugged, vegetated te rrain. Over 1200 fossils of terrestrial vertebrates have been recovered from the site of Lemudong’o L ocality 1 (LEM 1), which was formed at the edge of a shallow lake fed by slow-moving streams. Much lik e smaller Rift Valley lake basins in Kenya today, the Lemudong’o lake margin probably sup ported a mosaic of habitats ranging from closed riparian woodland to grassland and swamps. There are two fossiliferous horizons at LEM 1, clayey sands and gravels and overlying mudstones. Although the mudstones yielded the majority of the fossil mate rial, there are significant faunal differences between the two horizons. The mudstone assemblage con sists of taxa whose modern representatives primarily prefer relatively closed environment s such as riparian forests, as well as many species that prefer open woodland to wooded grasslands. The sa ndstone assemblage samples fauna from a wider range of habitats. This contrast in taxonomic co mposition suggests that the mudstone and sandstone horizons sample a lakeshore environment t hat was varying through time. The apparent shift in habitat preferences of the fauna is con sistent with the geological and geomorphological evidence for a mosaic of closed to open ha bitats that characterize rapidly variable rift-valley lake basins in mesic climatic regimes. One of the salient characteristics of these assemblages is the complete ab sence of fish, and the paucity of large mammals and reptiles, such as hippos, crocodiles, and lar ger bovid species that would be expected at the edge of lake basins fed by large rivers. Modern cent ral rift-valley lake basins that are fed by small streams vary widely in size and salinity in resp onse to climate change, and occasionally dry out completely. They do not contain fish and crocodil es, and only one has a substantial hippo population. These modern rift-valley lakes may there fore provide an analog to the depositional environment of Lemudong’o. The LEM 1 fossil assemblage is also unusual because it is dominated by small mammalian taxa, including numerous rodents, small colobine monkeys, hyracoids, and a div ersity of viverrid and other carnivores. Given the lack of evidence for fluvial transport in the m ain fossil horizon, the biased size composition, and the significant carnivore damage on the bone s, we interpret this site to represent an accumulation of carcasses by several avian and small mammali an carnivores. This paleoecological and paleogeographic reconstruction is discussed relat ive to penecontemporaneous fossil sites in Africa.