PALEOECOLOGY AND PALEOENVIRONMENT OF A RARE UPPER ORDOVICIAN (CINCINNATIAN; UPPER KATIAN) ECHINODERM OCCURRENCE

Multiple slabs of fossiliferous muddy packstone, collected in 2021 from a roadcut on US 68 near Maysville, KY, exposing the Upper Ordovician Bellevue and lower Corryville members of the Grant Lake Formation (Cincinnatian), have yielded several partially articulated echinoderms. These fossils include crinoids (Glyptocrinus sp.), four edrioasteroids, and over 30 semi-articulated cyclocystoids; this is one of the largest occurrences of this normally very rare echinoderm ever found. Associated fauna includes abundant ramose bryozoans (Parvohallopora sp.), the brachiopods Rafinesquina and Vinlandostrophia spp., and disarticulated molts of the trilobite Isotelus sp., a typical association for the Grant Lake Formation. The present study analyzed taphonomy, population density, associated fauna, and lithology in order to further elucidate benthic paleoecology, particularly that of the rare cyclocystoids. Sedimentology and taphonomy suggest time-averaged assemblage mixed with remains of a final living association, which was buried rapidly (hours to days), although not instantaneously. Sedimentologic and faunal features indicate a moderately shallow, mid shelf setting with non-indurated, shelly substrates. Probable taphonomic feedback, led to moderately diverse associations. While echinoderm skeletal debris is relatively common in the Grant Lake, articulated specimens of echinoderm crowns and tests are rare in this unit, as they require sudden burial to remain intact after death. All the cyclocystoids are missing the intricately plated central disk and experienced some distortion and partial disarticulation of the marginal disk and peripheral skirt. More complete preservation of crinoids and edrioasteroids in the same beds suggests that cyclocystoids may have been particularly fragile and largely disintegrated within hours after death. However, this occurrence suggests that cyclocystoids may have been relatively common on the shallow Cincinnatian seafloor but were only rarely preserved intact in shallow water settings, which they preferred. The preservation of these and other fragile echinoderms in particular appears to be the result of a storm event and associated mud burial layer, which fortuitously was not later reworked and preserved these normally rare echinoderms en masse.