BIOCHRONOLOGY, PALEOECOLOGY AND BIOGEOGRAPHY OF THE PLIO-PLEISTOCENE FOSSIL MAMMAL FAUNAS OF THE WESTERN RIFT (EAST - CENTRAL AFRICA) AND THEIR IMPLICATION FOR HOMINID EVOLUTION

The Western Rift Neogene fossil faunas may provide data that can test a number of crucial and long standing hypotheses in hominid evolution. An ecosystem approach to these problems is adopted here by studying the biochronology, biogeographic placement and paleoecological contexts of Plio-Pleistocene Western Rift mammalian faunas of the Kaiso Formation and Lusso Beds.; The mammal faunas of five major localities are updated taxonomically with emphasis on the systematics of the hippopotamids. The Lusso Beds hippopotamid material is classified into two taxa, cf. Hexaprotodon imagunculus and Hippopotamus amphibius (= Hippopotamus kaisensis). When morphological variation of a sample of extant Hippopotamus amphibius skulls is taken into account, Hippopotamus kaisensis is indistinguishable from the modern species and, therefore, a separate species status is not warranted.; The age of the five faunas estimated by the Concurrent Biochronological Range and Stage-of-Evolution methods indicated a possible time range from ca 3.5 to 1.8 MY, without any considerable gap between each fauna. It is possible that each represents a mixture of more than one time horizon. Application of the Computerized Temporal Biostratigraphy method (Shuey et al., 1978) gave best-fit dates of the faunas from 2.6 to 2.3 MY.; The low taxonomic diversity and the possibility that the faunas are chronologically mixed limits the potential of the paleoecological analysis. Based on indicator species, a series of environments are suggested for each fauna, including both forest and savanna habitats.; Study of geographical distribution around the Western Rift, and cluster analysis of 23 modern sub-saharan African mammal faunas based on Simpson's Faunal Similarity Index indicate that the Western Rift acts as an ecological filter. This filtering is attributed to the large variety of habitats present in the Rift, that result from the interaction of large scale climatological, topographical and geographical factors.; Application of the same analyses to other Plio-Pleistocene faunas in Africa, indicates that the Western Rift faunas do not cluster with East African faunas. This result could disprove the hypothesis of different provinciality for the Western Rift faunas, because of the possibility of sampling error.