GEOLOGY
Taphonomy of Late Ordovician cyclocystoids from the Millersburg Member, Lexington
Limestone, central Kentucky. NEIL E. RUSSELL* and FRANK R. ETTENSOHN,
Department of Earth & Environmental Sciences, University of Kentucky, Lexington, KY
40506.
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,
coarse-grained, 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 (Smith and Paul, 1982), 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.