Sampling and Faunal Turnover in Early Eocene Mammals

Badgley, C. and Gingerich, P. D., 1988. Sampling and faunal turnover in Early Eocene mammals. Palaeogeogr., Palaeoclimatol., Palaeoecol., 63: 141-157. Faunal turnovers in the fossil record are episodes of synchronous appearance and disappearance of species from a community, often resulting in net change in species richness. We studied the biostratigraphic record of faunal turnover involving early Wasatchian (early Eocene) mammals from the Clark's Fork Basin, Wyoming, U.S.A. Two faunal turnovers occur in this record one at the base of the Wasatchian, comprised mainly of appearances of taxonomically and ecologically distinctive species, and a later one, Biohorizon A of Schankler (1980), comprised mainly of disappearances, especially of carnivorous species. This study focuses on Biohorizon A. In the record of the Clark's Fork Basin, Biohorizon A may be an artifact of sampling. Sample size and species richness are highly correlated (r = 0.95) throughout this record. Moreover, sample size and species richness fluctuate markedly between successive stratigraphic intervals; peaks of appearances coincide with large sample sizes and peaks of disappearances with low sample sizes. The peaks and valleys in fossil productivity over time mask the real timing of appearances and disappearances of species. Changes in fossil productivity in the stratigraphic section may result from changes in exposure area, taphonomic factors, or ecological factors. Evaluation of the effects of sampling is a necessary prerequisite for investigating the chronological and ecological significance of faunal turnovers. Introduct ion Fauna l tu rnover s have been recognized over a considerable r ange of spat ial and temporal Fauna l t u rnove r refers to change in species scales. At a large scale, for example, Kitchel l composi t ion of a fauna t h r o u g h the disappearand Carr (1985) modelled diversif icat ion of and ance of some species and the appea rance of in te rac t ions among three Phanerozo ic mar ine o ther species. The d i sappearances may be local faunas (at the family level) as compiled by or global ex t inc t ions or may be only t empora ry Sepkoski (1981). At a small scale, for example, d isappearances; the appearances may be local Schank le r (1980) identified episodes of extincor global first appearances or else reappeart ion and immigra t ion wi th in the mammal ian ances of taxa t h a t were former ly present. The f auna for par t of the ear ly Eocene, wi th in par t fossil record abounds in fauna l turnovers , of a deposi t ional basin. At an in termedia te They mark m a n y chrono log ica l boundar ies in scale, for example, Webb (1984) described the geological t ime scale. The major faunal o r ig ina t ion and ex t inc t ion within the mammaltu rnovers those of broad t axonomic scope ian fauna of Nor th Amer ica dur ing the middle and wide geographic c o v e r a g e a r e impor t an t to late Miocene and also the in t e rchange of as events in ecological and evo lu t iona ry hismammals be tween Nor th and South Amer ica to ry and as guides to reg ional corre la t ion , dur ing the late Neogene. 0031.0182188/$03.50 © 1988 Elsevier Science Publishers B.V.