PALEOBIOGEOGRAPHIC CONNECTIONS BETWEEN THE AFRICAN RIFT LAKES: AN EXAMPLE FROM THE LAKE RUKWA BASIN AND IMPLICATIONS FOR THE INTRALACUSTRINE DISPERSAL OF SPECIES FLOCKS

Biogeographers and evolutionary biologists have long debated whether and how species become dispersed between apparently isolated lakes in the African rift valleys. Morphological similarities between species in separated lakes could be the result of active upstream/downstream dispersal through waterways that no longer exist, through passive dispersal (i.e. birds or other agents) without invoking a past hydrologic connection, or as a result of evolutionary convergence. Here we report on fossil evidence from the Lake Rukwa Basin, which indicates a former hydrologic connection with downstream Lake Tanganyika. At its maximum Early Holocene extent (968masl) paleolake Rukwa was an open basin ~300km long and ~200m deep, draining to Lake Tanganyika via the Karema gap. Early Holocene gastropod fossils from the Rukwa basin that closely resemble the modern Lake Tanganyika “endemic” genus Lavigeria have been known since the 1930s. Recently discovered fossil localities, from probable early Holocene high stand beach ridge deposits at the extreme north end of the basin, expand on the earlier findings and suggest considerable biogeographic complexity in paleolake Rukwa itself. A fossil assemblage from the south end of the basin (far from the probable spillway into the Tanganyika basin) contain fossil mollusks and ostracodes with close affinities to modern Tanganyika endemics, whereas apparently contemporaneous faunas from the north end of the basin (close to the spillway) are quite different, with more differentiated Lavigeria and only cosmopolitan but diverse ostracodes. North basin fossil assemblage taphonomy suggests these fossils accumulated in back-beach, vegetated lagoons, probably isolated from the main lake, whereas the south basin assemblages are more likely from the main lake itself. Tanganyikan-derived mollusks apparently persisted in the Rukwa basin into the mid-Holocene after the end of the Early Holocene humid period, long after it dropped below its spillway threshold. Based on the size and modern distribution of ”Tanganyikan” endemics, they most likely reached the Rukwa basin through active upstream dispersal.