SITE-SELECTIVE PARASITIC EMBEDMENT STRUCTURES ON MIDDLE SILURIAN CRINOID COLUMN MATERIAL FROM WESTERN ESTONIA

Crinoid specimens containing symbiotic (potentially parasitic) pits are relatively rare in Silurian strata of Estonia. The first specimen of middle Silurian-age (Wenlock: Sheinwoodian) crinoid material containing such pits—a pluricolumnal of uncertain affinity—was recently reported from the Jaagarahu Formation of Saaremaa Island. The presence and morphology of pits were previously described, but a noteworthy paleoecological phenomenon that was not initially documented involves the site-specificity of pit occurrence along the pluricolumnal. All 21 of the pits distributed around the lateral circumference of the specimen are located on plate sutures, demonstrating a degree of site-selectivity that contrasts strongly with the more random distribution of pits observed in most Silurian crinoid assemblages. The underlying paleoecological driver of this site-selectivity remains enigmatic, particularly considering some reports that suggest a preference for the latera rather than the sutures of Paleozoic crinoid columns.