The Effects of Spatial Patchiness on the Stratigraphic Signal of Biotic Composition (Type Cincinnatian Series; Upper Ordovician)

Patchiness affects fine-scale patterns of biological variation because compositional differences among localities depend on the composition of the patch sampled for paleoecological analysis. This can inflate compositional differences among localities that otherwise may have similar faunal constituents, which can obscure the true signal of biotic composition. In the type Cincinnatian Series, comparisons of fine-scale faunal-assemblage patterns acquired through gradient analysis have been difficult to accomplish because of significant deviations in biotic composition. Here, the role of patchiness in creating these deviations is tested by quantifying the amount of lateral variation in composition at a single outcrop of the Kope Formation. Because the lateral expression of individual beds in the Cincinnatian is variable, patchiness is more effectively assessed for bedsets rather than individual beds. Using gradient analysis to evaluate lateral variation at one locality reveals that patchiness is the cause of fine-scale stratigraphic deviations, which are typically produced by the addition or subtraction of just a few taxa. Furthermore, this lateral variability is less for limestone bedsets than for many mudstone bedsets. This supports the idea that limestone beds in the Cincinnatian contain assemblages that are more time-averaged than many, though not all, mudstone beds. Because fine-scale faunal patterns are dependent on the composition of the sampled patch, replicate sampling is necessary to account for patchiness. Despite the effects of patchiness and time-averaging at scales of one or a few beds, biotic composition patterns acquired through gradient analysis are robust among localities at scales greater than a few beds.