THE ROLE OF STRATIGRAPHIC SCALE IN THE ASSESSMENT OF BIODIVERSITY

The type Cincinnatian series of Ohio, Indiana and Kentucky has long been used as a proving ground for studying fine scale variation in biodiversity during the Upper Ordovician. Numerous studies of this interval have demonstrated a paleoecological depth gradient based on the presence and relative abundances of different taxa. Because of the apparent paleoecological fidelity that these rocks possess, it may also be possible to investigate the variation in biodiversity caused by sampling strategy, namely according to the resolution of stratigraphic sampling. Since some studies seek to characterize biodiversity of a region by amalgamating data from a locality or several localities to get an overall assessment of richness, this study investigates the way in which biodiversity varies according to stratigraphic scale. By combining strata into a number of binning schemes (e.g., bed-level, multiple beds, entire outcrop) it may be possible to determine how much of the variation in diversity is lost through this process of amalgamation. The dataset used here is derived from 108 bulk limestone, siltstone and shale samples collected from the Pioneer Valley member of the Kope Formation . Genus-level abundance counts were obtained, yielding nearly 5,000 fossil individuals.

The working hypotheses for this investigation are as follows: (1) the richness of fossil communities is directly related to the stratigraphic thickness of the interval considered, where larger intervals will yield greater assessments of biodiversity accounting for some degree of ecological variability over many generations; (2) the proportion of rare taxa within paleocommunities remains constant when assessed in stratigraphic intervals of equal sizes despite changes in ecology through time; and (3) the proportion of rare taxa present in a stratigraphic interval increases when assessed over increasingly coarse stratigraphic bins because more time is represented, permitting greater evolutionary and ecological variability. Through understanding how different properties of biodiversity relate to stratigraphic scale, we may gain insight into the stratigraphic resolution appropriate for characterizing the biodiversity of an interval.