TRILOBITE OBRUTION HORIZONS WITH “FROZEN BEHAVIOR”: PALEOBIOLOGICAL INSIGHTS FROM TAPHONOMIC AND ECOLOGICAL WINDOWS

Extraordinarily preserved trilobites, indicating episodic mass mortality and obrution in fine-grained sediments, occur preferentially in offshore mudrock facies of Middle Ordovician to Late Devonian age and provide extraordinary insights into the paleobiology of these ancient organisms. Distinctive taphofacies of articulated trilobite bodies and molts occur in: a) dysoxic, pyritic shale facies, and b) more abundantly fossiliferous, burrowed calcareous mudstones, and interbedded concretionary limestones. Diagenetically enhanced specimens are uncompressed and commonly occur in distinctive orientations, including probable life positions. Carbonate cementation and/or pyritic infilling occurred during interludes of low sedimentation within the zone of sulfate reduction, prior to compaction. Pyritic and calcite spar fillings in internal spaces of fossils formed early and rarely preserve remnants of lightly sclerotized tissues providing insights into soft part biology. The occurrence of beds rich in molt ensembles not only illustrates modes of exuviation, but also proves in situ preservation and therefore living sites of trilobites. High proportions of enrolled vs. outstretched trilobites, especially in pyritic beds, indicate defensive behavioral response to external stimuli associated with storm-generated disturbance in marginal environments prior to burial. Species-segregated carcass and molt clusters of similar-sized trilobites point to mass-molt-mate behavior. Finally, the recent discovery of rows of more than a dozen specimens, all oriented with cephala facing in the same direction and facing or overlapping the pygidia of the next forward individuals, provide the oldest evidence of migratory queues similar to those seen in modern crustaceans. Such evidence points to complex synchronized escape and reproductive behavior in trilobites. Abundance of such clustering in particular distal facies suggests that these record “snapshots” of distinctive environmental windows prone both to trilobite aggregation and mass mortality/burial.