ORDOVICIAN TRILOBITES GETTING UNDER "DINOSAUR SKIN": COMPLEX PRESERVATION OF A MICROBIAL MAT (?) IN OFFSHORE SILICICLASTIC MUDSTONE AND CARBONATE FACIES: KOPE FORMATION (UPPER ORDOVICIAN), KENTON COUNTY, KENTUCKY, USA

Unusual preservation of small primaspid trilobites may provide a key to the first recognition of a microbially induced sedimentary structure in the subtidal U. Ordovician Kope Fm. of northern Kentucky. A thin, elliptical lens of silty mudstone (2 m long, at least 1 m wide, and up to 3 cm thick) in the lower Kope Formation (Pioneer Valley Submember, McMicken Member, C-1 Sequence) has several unique features mainly restricted to its basal surface. Toward one end the basal surface has parallel-fluting that terminates in a complex of conjoined, flattened, ellipsoidal or spatulate concretion-like lobes at the other end. The entire basal surface has small-scale wrinkles or corrugation (3-4 mm wide) variably developed. Articulated carapaces of small, spinose primaspid trilobites (Primaspis crosota) occur with their ventral surfaces applied directly to the wrinkled surfaces (i.e. dorsal-side-down). The wrinkled texture of the basal surface is aligned with the minor axis of the elliptical lobes and can wrap around to their upper surfaces. At the rounded ends of the lobes the wrinkling becomes reticulated by crosscutting of wrinkle sets. In the same region the reticulate undersurface is covered with minute rounded pustules (~0.3 mm). Other Cincinnatian occurrences of primaspids on undersurfaces of bryozoans and brachiopods suggest a cryptic life habit for purposes of feeding or shelter. This habit, together with the wrinkled, reticulate and pustulose undersurface suggest that a cohesive membrane or mat, most likely of microbial origin, was smothered by sediment influx triggered by storm or seismic disturbance. Evidence of the mat is preserved as an impression in hyporelief, like some dinosaur skin. Loading and lateral shifting of the sediment-laden mat resulted in wrinkling, trapping of fluid sediment, and convolution of boudinage-like lobes. As an alternative, the lobate structures suggest infilling of a 3D, sacklike organism (?) flattened by compaction, with "wrapping" of the corrugation onto the upper as well as basal surface. Whatever the origin, the unique combination of features reported here has never before been found in the Kope Fm., a subtidal lithofacies that has received intense scrutiny.