| dc.description.abstract | The Lemudongo Formation in the Narok District of Kenya comprises a 135-mthick
series of predominantly lacustrine and lake basin margin sedimentary rocks with
interstratified 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
western 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
terrain. Over 1200 fossils of terrestrial vertebrates have been recovered from the site of
Lemudongo Locality 1 (LEM 1), which was formed at the edge of a shallow lake fed by slow moving
streams. Much like smaller Rift Valley lake basins in Kenya today, the Lemudongo lake
margin probably supported 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 material,
there are significant faunal differences between the two horizons. The mudstone assemblage
consists of taxa whose modern representatives primarily prefer relatively closed environments
such as riparian forests, as well as many species that prefer open woodland to wooded
grasslands. The sandstone assemblage samples fauna from a wider range of habitats. This
contrast in taxonomic composition suggests that the mudstone and sandstone horizons sample a
lake shore environment that was varying through time. The apparent shift in habitat preferences
of the fauna is consistent with the geological and geomorphological evidence for a mosaic of
closed to open habitats that characterize rapidly variable rift-valley lake basins in mesic climatic
regimes. One of the salient characteristic of these assemblages is the complete absence of fish,
and the paucity of large mammals and reptiles, such as hippos, crocodiles, and larger bovid
species that would be expected at the edge of lake basins fed by large rivers. Modern central rift-
valley lake basins that are fed by small streams vary widely in size and salinity in response to
climate change, and occasionally dry out completely. They do not contain fish and crocodiles,
and only one has a substantial hippo population. These modern rift-valley lakes may therefore
provide an analog to the depositional environment of Lemudongo. 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 diversity of viverrid and other carnivores. Given the
lack of evidence for fluvial transport in the main fossil horizon, the biased size composition, and
the significant carnivore damage on the bones, we interpret this site to represent an accumulation
of carcasses by several avian and small mammalian carnivores. This paleocological and
paleogeographic reconstruction is discussed relative to penecontemporaneous fossil sites in
Africa. | en |
