Paleobiologic and taphonomic aspects of the “granulosa” trilobite cluster, Kope Formation (Upper Ordovician, Cincinnati Region)

Abstract

A new, low-density trilobite body cluster from an Upper Ordovician Kope Formation mudstone in the Cincinnati region is characterized by well-preserved, articulated specimens. This cluster is polytaxic, size-segregated, and shows an unusual variety of flexure postures in the most common taxon,Flexicalymeneaff.granulosa.Specimens show internal pyritization and associated pyritefilled tubular burrows, and were preserved by rapid deposition of mud from a sediment-laden flow. Size-segregation among species, but size-consistency within species, coupled with sedimentological data and apparently haphazard body orientation data, suggest that the trilobites represent a biological association, rather than the product of mechanical sorting. While the majority of specimens are inverted, many are dorsal-up, and some are inclined to bedding. Postures include prone (i.e., nonflexed), simple dorsal or ventral flexure, and complex stances that combine both dorsal and ventral flexure. Flexure pattern appears largely unrelated to sediment compaction. We interpret the cluster as an association of live trilobites that were entombed in situ. Specimens may have inverted prior to burial as a behavioral posture, or as a response to onset of burial. The animals were likely preserved in the process of escaping, as they tried to free themselves from being buried. This is in contrast to the tightly enrolled stance in some trilobites, especially otherFlexicalymene, commonly associated with rapid burial events, and indicates a new behavioral aspect of trilobite paleobiology. The distribution of associated burrows suggests that the rotting carcasses attracted scavengers.