FALSE CRINOID COLUMN IMPRESSIONS IN THE LOWER SILURIAN ESTILL SHALE OF CENTRAL KENTUCKY

Unusual specimens of the ichnofossil Arthrophycus are described from siltstone tempestites in the Lower Silurian (Llandovery: Telychian) Estill Shale near Irvine, Estill County, central Kentucky, USA. The specimens display closely spaced, uncurved transverse annulations but lack the medial ridge, rectangular cross-sectional shape, and dense branching common to, but not diagnostic of, Arthrophycus. This combination of features, coupled with the straightness of burrows and alignment parallel to inorganic tool marks, closely approximate the appearance of partially articulated crinoid columns that were impressed into firm mud and cast by storm-deposited sediment. That these structures do not represent the external molds of body fossils is suggested by minor width variations along the long axis of specimens, possible bioglyphs, and the total absence of skeletal material, including isolated crinoid columnals, in this and other tempestites from this interval. More convincingly, the presence of very sparse branching indicates a trace-fossil origin for these structures. These fossils are best interpreted as the products of preferential mining of inorganically generated tool marks by deposit feeders, owing to exposure of organic-rich muds by erosional scouring. Thus, this occurrence represents a rare instance where trace fossils display a strong alignment parallel to inorganic sedimentary structures.