Clustered trilobite assemblages in the Middle Devonian Hamilton Group

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Abstract
Clusters of well-preserved trilobites occur throughout lower and middle Paleozoic strata. This phenomenon remains poorly understood. Several thin, regionally persistent beds in the Middle Devonian (Givetian) Hamilton Group of New York State [USA] display taphonomic and sedimentary features which indicate rapid burial by storm-generated clouds of fine-grained sediment. Fossil components in these units display little or no post-mortem disturbance and reflect in situ faunal associations. These beds contain 2 types of clustered assemblages of the trilobites Phacops rana (Green), Greenops boothi (Green) and Dechenella rowi (Green). Body clusters comprise dense assemblages of 3 or more complete trilobite corpses. Molt clusters are composed of 3 or more molt ensembles; i.e., articulated thoracopygidia in close proximity to corresponding cephala. Clusters are species-segregated and contain only a small portion of the total size range exhibited by these trilobite species. Body clusters and molt clusters are morphometrically undifferentiated indicating that these 2 cluster modes represent related phenomena. Analogy with modern marine arthropods suggests that trilobites assembled into monospecific, age-segregated clusters and molted prior to en masse copulation.