FOSSIL PRESERVATION IN AFRICAN RIFTS - BIASED SAMPLES OF EVOLUTION ON A LARGE CONTINENT?

Africa is characterized by major rift systems that have been active throughout the Phanerozoic, providing depositional sumps as well as physiographic corridors across the eastern, southern, and central portions of the continent. Sedimentary deposits in rift basins account for a large proportion of the African terrestrial fossil record. Does fossil accumulation in these rifts provide an unbiased sample of what was happening on the continent as a whole, or do rift systems represent an environmentally and taphonmically delimited view of Phanerozoic evolution in Africa? Answering this question requires comparisons of faunas and floras from rift and non-rift settings and consideration of environmental as well as taphonomic processes relating to rift tectonics. Depositional environments in rift settings are dominated by fluvial systems and lakes along the rift axis that may extend for 1000s of km across major latitudinal climate zones. This axial continuity contrasts with topographic and climatic variability (ranging from desert to rain forest) perpendicular to the rift axis, over distances of 10’s to 100’s of km. Rifts thus provide macro-ecological settings that differ markedly from those of craton, foreland basin, and passive continental margins. Rapid sedimentation and permanent burial are key to preserving high quality floral and faunal records. Rift tectonics control accommodation space and sediment input, and sedimentation rates can exceed 2 m/kyr, providing high resolution records of environmental conditions and biotas for particular time intervals (e.g., 10-1000 kyr). Climate exerts a strong secondary control on fossil preservation in rift basins; warm and dry conditions and carbonate-rich environments promote vertebrate preservation, while wetter conditions are conducive to plant preservation. Volcanism has provided added sediment influx in the Cenozoic of East Africa, increasing preservation rates and resulting in the unusually rich vertebrate fossil record associated with human evolution. The ecological, depositional, and taphonomic features of African rift systems have resulted in a paleobiological record that, while likely biased relative to the continent as a whole, also represents a long-lasting and unique evolutionary theater.