An assemblage of blind phacopid trilobites of the genus TrimerocephalusMcCoy, 1849, representing either the species Trimerocephalus mastophthalmus (Reinhard Richter, 1856) or its allies (possibly, a new species), from an Early Famennian (Early Marginifera Zone) marly sequence of the Holy Cross Mountains, Central Poland, is composed of well organized single-file queues. The trilobites in the queues appear almost intact, having been preserved in the position they kept when forming the queues, and are interpreted showing migratory behaviour known in various present-day arthropods, but unreported from the fossil state. This queuing was induced by environmental stress caused by a dramatic drop in sea level, temporarily leading to emersion. The preservation of the queues at omission horizons is thus ascribed to a mass mortality event, caused by subaerial exposure. The trilobites were suffocated and fossilized in a mortal spasm, and finally blanketed by calcareous ooze when inundated at a highstand. The assemblage of trilobite queues represents a unique example of frozen behaviour and a snapshot of the geological past.
