The results of this study indicate that marine zones in the Glenshaw Formation contain four recurrent biofacies that are positioned along an environmental gradient that represents changing conditions associated with nearshore to offshore settings. These four biofacies re-appear with a distinctive composition-abundance structure tracking a preferred set of environmental conditions related to glacio-eustatic sea-level changes in the Appalachian Basin. A symmetrical (Nearshore-Offshore-Nearshore) pattern is found in only one marine zone, whereas, the asymmetrical (Offshore-Nearshore) biofacies pattern found in three of the four marine zones is attributed to relatively rapid rates of transgression commonly associated with glacio-eustatic cycles.
The eight most abundant genera were non-randomly distributed among the four Glenshaw biofacies, indicating a consistent environmental preference. In contrast, only four of the remaining 15 less abundant genera were non-randomly distributed. The abundant taxa maintain a more consistent membership among the four biofacies by tracking their preferred environment. An inherent aspect of environmental tracking is that species respond independently, leading to variations in the taxonomic structure of faunal assemblages. The fact that nearly half of the genera that compose the four Glenshaw biofacies are randomly distributed illustrates this variation.
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