Is Africa Really an “Odd Man Out”? Evidence for Diversity Decline across the Oligocene-Miocene Boundary

Abstract

Premise of research. Africa was declared the "odd man out" by P. W. Richards because its tropical forests have low species diversity and endemism relative to the Neotropics and Southeast Asia. A variety of hypotheses, including differences in climate, the geographic extent of aseasonal warm and wet conditions, landscape heterogeneity, historical contingencies, and a combination of these factors, have been proposed to explain this pattern. New paleofloral data are necessary to document when Africa became the "odd man out" and, when combined with paleoclimate reconstructions, can be used to test these hypotheses. Here, we present floral diversity and paleoclimate analyses from two sites on the Ethiopian Plateau that cross the Oligocene-Miocene boundary, hypothesized to be an interval of diversity decline in tropical Africa. Methodology. Floral diversity in the late Oligocene Chilga Basin and the early Miocene Mush Valley was quantified using large census collections from each site. Diversity at these sites was also compared with that of modern leaf assemblages from well-studied tropical forest plots. Mean annual precipitation was estimated for each fossil site using leaf area analysis, and additional climate variables were reconstructed for Chilga using an overlapping distribution analysis. Pivotal results. Floral diversity at Chilga was extremely high (comparable to that of modern South American leaf assemblages) and decreased at Mush. The Chilga paleoclimate was very humid and possibly aseasonal, on the basis of paleobotanical reconstructions and the presence of lignites. Conclusions. Chilga is among the most diverse fossil floras ever analyzed. The significant decline in Ethiopian tropical forest diversity observed here supports previous hypotheses of significant extinction in the African tropics across the Oligocene-Miocene boundary. We hypothesize that increased seasonality since the Oligocene is the first-order factor responsible for low plant diversity in the modern African tropics relative to South America.