BIOTIC EVOLUTION: ISLAND AFRICA AND VERTEBRATE FAUNAL DYNAMICS

Africa serves as an evolutionary experiment highlighting interactions among biogeographic, ecological, and biotic processes. Africa became an island continent in the Cretaceous (with Arabia attached); it remained so until the Oligocene-Miocene boundary, when it docked with the northern continents, closing the Tethys Sea. This isolation of Africa gives an independent view of biotic change during global transitions. Events such as the Cretaceous-Tertiary extinctions, marked by a transition from dinosaurs to mammals, occurred in Africa independently of parallel transitions in other regions, confirming that global conditions led to common outcomes on different landmasses. Most African mammals evolving during the island period came from a eutherian ancestor that lived in dinosaurian Africa, but which radiated only after the K-T boundary. From this endemic Afrotherian ancestor, Africa evolved tiny insect-eaters to graviportal herbivores. Overwater immigration of mammals into Africa occurred, but at low rates, given the vast time intervals. Non-Afrotherians that arrived in the Paleogene (“archaic African groups”) include primates of several lineages, creodonts, phiomyid rodents, and anthracotheres. A typical mammal community in Early Tertiary Africa consisted of herbivorous hyracoids and proboscideans, with creodonts as essential meat-eaters, and primates as arboreal frugivores. Recent field research in the Oligocene shows this community of isolates persisted until 27 ma at Chilga, Ethiopia, and until 24 ma at Nakwai, Kenya, just before the Oligocene-Miocene boundary (23.03 ma). This Early Tertiary fauna was replaced during a startlingly short interval by northern immigrants in the early Miocene, including such classical “African” mammals as rhinos, zebras, tragulids, warthogs, antelopes, giraffes, and rodents. Whether this faunal replacement was due to competition or other evolutionary mechanisms is under study. Evolutionary patterns of birds and plants may have followed different trajectories than those of mammals. The dynamic transition between Paleogene and Neogene communities are a focus of research; we hope to address how such fundamental changes correlate with climate, environment, plant communities, mammalian fauna, and biogeography.