Testing patterns and causes of faunal stability in the fossil record, with an example from the Pliocene Lusso Beds of Zaire

Abstract

Abstract Two central issues emerge from recent discussions of Coordinated Stasis in the fossil record: testing pattern and testing causality. Testing the generality of Brett and Baird's observations of faunal stability in the Paleozoic Appalachian Foreland Basin is important to determine whether such clustering of evolutionary and ecological change into relatively brief intervals of time is a general phenomenon in the fossil record or a quirk of that particular time and place. Methods for assessing the linkage between paleoecological and evolutionary change are discussed using examples drawn from the molluscan faunas of the Pliocene Edward-Mobutu basin of Zaire and Uganda. In the Late Pliocene Lusso Beds of Eastern Zaire an interval of paleoecological and morphological stability appears to be terminated by an episode of coordinated change. This paper illustrates a quantitative approach to simultaneously testing for the presence of coordinated change in paleoecological composition and species morphology. With due attention to taphonomic and stratigraphic controls, testing for the generality of faunal stability in the fossil record is a relatively straightforward exercise. Causality, however, is a thornier issue. Three broad classes of explanation for the presence of coordinated faunal change in the fossil record can be identified in the literature. These span a continuum from the purely evolutionary, through combined evolutionary/ecological to purely ecological explanations. I argue here that the critical test to distinguish between different causal hypotheses for coordinated faunal stability and turnover is an examination of correlation between environmental change and biotic change in the fossil record.