Mysterious large crinoid stems of the Late Mississippian Slade Formation of Kentucky (USA) probably belong to Rhabdocrinus

Donald R. CHESNUT*

University of Kentucky, 228 MMRB, Lexington, KY 40506, USA; chesnut@uky.edu

Since the early twentieth century, geologists in Kentucky have noted the abundant occurrences of large 
crinoid stems in the Late Viséan limestones of the Slade Formation (e.g. Butts, 1922). The large stems 
are so common on bedding planes of oolitic grainstones of the Tygarts Creek Limestone (=Reelsville-
Beech Creek Limestone), that they were used as marker beds (McFarlan et al, 1955, McFarlan and 
Walker, 1956) to aid in correlation. The stems are as wide as 3 cm in diameter, are round in outline 
and have highly crenulated sutures. The segments are typically 2 cm or longer in length. One specimen 
on a bedding plane was approximately 1.5 meters in length without any trace of root, crown, or change 
in diameter. The stems are unornamented. Large cirri (approximately 1 cm diameter) have been 
observed in one large root structure. The remnant root structure was not in situ and many of the cirri 
were missing. The round stems appear to be from only one type of crinoid. The large diameter stems, 
the 1.5 m-length of one stem and its constant diameter seem to indicate that the crinoid crown was 
also very large. A survey of literature for coeval Late Viséan crinoids along the Appalachian Basin 
revealed only one large crinoid with a stem of this size. That crinoid was Rhabdocrinus largus 
Burdick and Strimple 1982 [1983]. Rhabdocrinus belongs to the monogeneric Rhabdocrinidae, a 
Poteriocrinitacean (Order Cladida). This enigmatic genus (only one crown has been found in North 
America) has no known close relatives, and, therefore, its phylogeny is not known. The large-crinoid-
stem beds in the Slade Formation of Kentucky should be re-examined for disarticulated plates of 
Rhabdocrinus. If the identification is confirmed, the occurrence of the beds in Kentucky may shed 
more light on the paleoecology of this genus and would increase its significance as a member of the 
Late Viséan fauna of North America.

References
Burdick, D.W. and Strimple, H.L., 1982 [1983], Genevievian and Chesterian crinoids of Alabama. 
Geological Survey of Alabama, Bulletin 121, 277 p.
Butts, C., 1922, The Mississippian Series of Eastern Kentucky. Kentucky Geological Survey, Series 
VI, [Geologic reports], 188 p.
McFarlan, A.C., Swann, D.H., Walker, F.H., and Nosow, E., 1955, Some old Chester problems--
Correlations of lower and middle Chester formations of western Kentucky. Kentucky Geological 
Survey, Series IX, Bulletin 16, 37 p.
McFarlan, A.C., and Walker, F.H., 1956, Some old Chester problems--Correlations along the 
eastern belt of outcrop. Kentucky Geological Survey, Series IX, Bulletin 20, 36 p.