The evolution of biodiversity among the Southwest European Neogene rodent (Mammalia, Rodentia) communities: pattern and process of diversification and extinction
Abstract The history of Southwest European Neogene rodent communities is analyzed in light of species diversity, immigration and extinction rates. The distribution in time of the 285 extinct species indicates that species-diversity changes occurred in parallel with the oxygen isotope marine curves and suggests a relationship between oceanic mean temperature and terrestrial mammals species diversity. Extinction rates also seem to coincide with environmental changes observed in the oceans. Three high extinction-rate periods can be recognized, at 15 Ma and between 3.3 and 2.4 Ma. Nested polycohort survival curves indicate an overall constant extinction rate interrupted by sudden bursts of higher extinction rates, which correspond to the periods of high extinction rates previously identified. Polycohort analysis indicates an increasing extinction rate through time, which corresponds also to a decreasing mean value of species duration, tentatively attributed to an increasing frequency of climatic changes. The survival curve for all species during the entire time interval is linear, demonstrating a constant extinction rate. Increased extinction rates during some short time intervals therefore do not effect the overall constant extinction rate and do not modify the prediction of the “Red Queen” hypothesis. This constancy of the extinction rate is used to recalibrate the biochronologic time scale. The results cannot be rejected on the basis of the available data, and represent a robust test for the hypothesis of constant extinction rate in mammalian communities. Bursts of increasing boodiversity correlated mainly with paleogeographic changes. The “proboscidean datum” at 18 Ma corresponds to a major faunal dispersion from the Middle East and Central Asia toward Western Europe, initiated by the closure of the Tethys sea. A second faunal dispersion from a Northest African centre across the Western Mediterrenean occurred between 7 and 6.5 Ma The third main event of increased diversity is contemporaneous with the Early Pliocene marine transgression. During Middle and Late Pliocene, the rodent faunas show a high turn-over rate, which is clearly related to the onset and strengthening of the glacial-interglacial cycles of the Northern Hemisphere. More detailed analysis and evaluation of the time-lag effects would need an increased precision in the calibration of the biochronological scales that are currently used.