University of Kentucky

Taphonomy of Late Ordovician Cyclocystoids from the Millersburg Member, Lexington Limestone, Central Kentucky

Presenter Information

Neil Russell, University of Kentucky

Institution

University of Kentucky

Abstract

Eight specimens from the very rare echinoderm class Cyclocystoidea were recovered on the base of a bed from a former outcrop along I-64 near Winchester, Kentucky. Although no longer extant, the outcrop exposed nodular limestones and shales and a few, coarsegrained, through-going limestone beds in the Millersburg Member from uppermost, Edenian parts of the Lexington Limestone. Based on marginal ossicle count (40-60), disc percent of test (83%-86%), test diameter (16-50 mm), the circle-to-ovoid skeletal morphology, and presence in Edenian rocks, the specimens most likely represent the species Polytryphocycloides depressus. Occurrence of this species in central Kentucky may possibly expand its known geographic range. Cyclocystoids are typically encrusters, found on the tops of firm- or hardground beds. In this occurrence, however, the specimens occur on the base of one of the through-going beds. Notably, the cyclocystoids are part of a basal, lag-like concentration with bryozoans, brachiopods, and gastropods in rocks that exhibit subtle planar cross-bedding and graded bedding, features that are indicative of shallow, open-marine storm deposits in the Lexington and Millersburg. The cyclocystoids display taphonomic evidence of transportation and deposition, including contorted and fractured marginal rings, misaligned and missing ossicles, thecae draped over other fossil shells, and possible upside-down orientation. Such a transported fossil assemblage containing cyclocystoids is a heretofore unreported phenomenon and implies that the organisms were gregarious, as well as the nature of the community and environment in which they lived. Previously unknown aspects of hydrodynamic stability, thecal strength, and the nature of attachment are also suggested by the occurrence.

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Taphonomy of Late Ordovician Cyclocystoids from the Millersburg Member, Lexington Limestone, Central Kentucky

Eight specimens from the very rare echinoderm class Cyclocystoidea were recovered on the base of a bed from a former outcrop along I-64 near Winchester, Kentucky. Although no longer extant, the outcrop exposed nodular limestones and shales and a few, coarsegrained, through-going limestone beds in the Millersburg Member from uppermost, Edenian parts of the Lexington Limestone. Based on marginal ossicle count (40-60), disc percent of test (83%-86%), test diameter (16-50 mm), the circle-to-ovoid skeletal morphology, and presence in Edenian rocks, the specimens most likely represent the species Polytryphocycloides depressus. Occurrence of this species in central Kentucky may possibly expand its known geographic range. Cyclocystoids are typically encrusters, found on the tops of firm- or hardground beds. In this occurrence, however, the specimens occur on the base of one of the through-going beds. Notably, the cyclocystoids are part of a basal, lag-like concentration with bryozoans, brachiopods, and gastropods in rocks that exhibit subtle planar cross-bedding and graded bedding, features that are indicative of shallow, open-marine storm deposits in the Lexington and Millersburg. The cyclocystoids display taphonomic evidence of transportation and deposition, including contorted and fractured marginal rings, misaligned and missing ossicles, thecae draped over other fossil shells, and possible upside-down orientation. Such a transported fossil assemblage containing cyclocystoids is a heretofore unreported phenomenon and implies that the organisms were gregarious, as well as the nature of the community and environment in which they lived. Previously unknown aspects of hydrodynamic stability, thecal strength, and the nature of attachment are also suggested by the occurrence.