STORM-BED TAPHONOMY OF LATE ORDOVICIAN (EDENIAN) CYCLOCYSTOIDS FROM THE MILLERSBURG MEMBER, LEXINGTON LIMESTONE, CENTRAL KENTUCKY

Nine specimens of the rare echinoderm class Cyclocystoidea were recovered on the base of a bed from a former outcrop along I-64 near Winchester, Kentucky. The outcrop, no longer extant, exposed nodular limestones and shales and a few, coarse-grained, through-going limestone beds in the Millersburg Member in uppermost, Edenian parts of the Lexington Limestone; the fossils were collected from one of these thick, coarse-grained beds. Based on marginal-ossicle count, disc percent of test, test diameter, circular-to-ovoid test morphology, and presence in Edenian rocks, the specimens likely represent the species Polytryphocycloides depressus, and occurrence in central Kentucky probably expands its known geographic range.

Cyclocystoids were typically encrusters on top of firm- or hardground beds. In this occurrence, however, specimens occur at the base of a through-going bed. Notably, the cyclocystoids are part of a basal, lag-like concentration with bryozoans, brachiopods, trilobite fragments, and gastropods in rocks that exhibit subtle planar cross-bedding, abraded convex-up shells, and graded bedding, features that are indicative of shallow, open-marine storm events in the Lexington Limestone. The cyclocystoids display taphonomic evidence of transportation and deposition, including contorted and fractured marginal rings, misaligned and missing ossicles, missing marginal-ring and disc plates, thecae draped over fossils, and upside-down orientation. Apparently, the cyclocystoids were ripped up and transported with other shell debris from a shoal-like setting and deposited from a concentrated flow as coarse basal parts of a migrating sand sheet in a more basinal setting. Transported fossil assemblages containing cyclocystoids are heretofore unreported and suggest a formerly gregarious life style in a community of largely sessile invertebrates. Previously unknown aspects of hydrodynamic stability, thecal strength, and the nature of attachment are also suggested by the occurrence.