Abstract

Cluster analysis reveals eight biofacies during a time of faunal turnover and regional oceanographic change in the middle Upper Ordovician of central Kentucky. These biofacies are arranged along a deep-water to shallow-water gradient, with the dalmanellid and the Sowerbyella biofacies in offshore facies, the Rafinesquina, the atrypid, and the ramose trepostome biofacies in deep-subtidal to shallow-subtidal facies, the Constellaria-Cyclonema and the Rhynchotrema biofacies in the shallow-subtidal facies, and the Solenopora-Hebertella biofacies in sand-shoal facies. These biofacies are not discrete, but rather they share a large number of taxa suggesting that they are arbitrary subdivisions of a depth-related ecological gradient. Nonetheless, they are useful as a point of comparison with other studies of this time interval in the eastern United States. Previous lower and middle Upper Ordovician biofacies studies portray a similar pattern of plectorthines and rhynchonellaceans in the shallow subtidal, strophomenids in the deep subtidal, and dalmanellids and plectambonitaceans in the offshore, indicating an overall temporal consistency to many of these biofacies at suprageneric taxonomic levels. Similarly, values of preferred depth, depth tolerance, and peak abundance of most taxa are generally conserved, and this is particularly true for abundant taxa. Nonetheless, this 2-m.y. record also indicates temporal changes in the occurrence of some biofacies, in the relative abundances of taxa in some biofacies, in the species membership within biofacies, and in values of preferred depth, depth tolerance, and peak abundance for several taxa. Collectively these indicate a dynamic aspect to an ecosystem that otherwise displays several attributes of stability and underscore that ecologic stability should not be viewed as a simple dichotomy between stability and instability. The dynamic aspects of this ecosystem suggest that these biofacies were not governed by extreme degrees of species interactions that resulted in near constancy of biofacies structure. Continuous changes in water temperature, nutrient supply, and turbidity during this time may have been a contributing factor to changes within this ecosystem.

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