Line up! Gregarious linear clusters of the trace fossil Lingulichnus from the Upper Devonian Catskill Formation, Pennsylvania, USA

Funnel-shaped trace fossils from marginal marine deposits of the lower Catskill Formation in Tioga County, Pennsylvania (Famennian, Upper Devonian) occur in unusual gregarious clusters, with burrows organized in linear chains along bedding planes. Burrows are preserved as casts on the bases of beds, in full relief within beds, and as filled burrow apertures on the tops of beds. Many burrows are stacked as a result of equilibration to sediment deposition, and clusters of tracemakers maintained their linear association while burrowing upwards following sedimentation. By analogy with modern examples, the linear alignment might improve feeding efficiency in tidally influenced habitats. More speculatively, the clusters might improve the efficacy of upward burrowing in response to burial on crowded bedding planes. The burrows are assigned to Lingulichnus verticalis, which is commonly attributed to lingulide brachiopod tracemakers. Lingulide body fossils are found in these strata, though not directly associated with burrows.