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

Abstract

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 finescale 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.

Heterogeneity in Geochemical Expression of Subaerial Exposure in Limestones, and Its Implications for Sampling to Detect Exposure Surfaces

“…Bennington (2003) likewise found that species abundance can be highly variable in replicate samples within single outcrops of one bed. The results of Bennington (2003) and Webber (2005), combined with the results of this study, suggest that many parameters measured from layered rocks, whether they be paleobiological, geochemical, or otherwise, may have much unexpected patchiness, and that replicate sampling along layers may be necessary to characterize such parameters and to yield lateral and temporal patterns not obscured or altered by the noise caused by patchiness.…”

“…In an analogous paleobiological study using replicate samples along stratigraphic horizons, Webber (2005) found much variation in fossil assemblages within bedsets, and he attributed that variation to faunal patchiness, which introduced noise into gradient analysis and interpretation of time series. Bennington (2003) likewise found that species abundance can be highly variable in replicate samples within single outcrops of one bed.…”

Analysing the representativeness of local‐scale palaeodiversity measurements: a case from the Lower Cretaceous plant assemblage of Hautrage (Mons Basin, Belgium)

“…These results reinforce the conclusions from previous studies about the importance of lateral sampling in palaeoecological studies (Lafferty et al . ; Bennington ; Webber ; Bonelli et al . ; Zuschin et al .…”

“…This irregularity can result in a loss of resolution, changing interpretations or masking important information such as community gradients (Lafferty et al . ; Webber ; Zuschin et al . ).…”

“…In recent years, several publications have dealt with the development and application of methods to extract reliable and representative conclusions from palaeoecosystems (Lafferty et al . ; Bennington & Rutherford ; Bennington ; Leighton & Schneider ; Webber ; Bonelli Jr et al . ; Zambito et al .…”