Abstract

In south-central Oklahoma, the Pooleville Member of the Bromide Formation is a shallowing upward sequence recorded by distal to more proximal storm-influenced shelf facies. The deepest subtidal facies contains horizons of dense clusters of a single species of isoteline trilobite, “Homotelusbromidensis Esker. Two horizons were excavated and oriented slabs removed for detailed study. The lower horizon is dominated by exoskeletons that have librigenae and hypostomes in place and are interpreted as carcasses rather than molts. The upper horizon is composed largely of molts, as indicated by the opening of the facial sutures and, in many individuals, separation of the cranidium from the remainder of the exoskeleton. In both horizons, facing directions of the exoskeletons are random, and the majority of specimens are in convex-up orientations. All specimens are holaspids and encompass a narrow size range. The trilobite horizons immediately are overlain by sparsely fossiliferous lime mudstone or thin, barren clay layers. This suggests that catastrophic mud blanketing, probably during major storms, preserved the horizons. Thus, the Ordovician clusters represent snapshots of trilobite behavior. As with younger clusters of phacopid trilobites described from the Devonian of New York State, they most likely record behavioral aggregation of individuals for synchronous molting and reproduction. The occurrence of clustering in species from such phylogenetically distant clades as the Asaphina and Phacopida indicates that the aggregation is likely to have been a behavioral characteristic of trilobites in general. These assemblages also provide insight into the depositional dynamics in a otherwise monotonous interval of lime mudstones. They demonstrate an episodic pattern of sedimentation that would be overlooked without taphonomic analysis.

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