EXAMPLES OF INTER- AND INTRASPECIFIC INTERACTIONS IN MARINE INVERTEBRATES FROM THE FAMENNIAN (UPPER DEVONIAN) OF PENNSYLVANIA

Understanding interactions among organisms is vital for understanding the history of life, as discussed by Signor and Brett in their classic mid-1980s paper, “The mid-Paleozoic precursor to the Mesozoic marine revolution” (Paleobiology, 10:229-245). Here, we discuss two new examples of interactions recorded by fossils from the Famennian of Pennsylvania.

First, a crinoid preserved inside a hexactinellid sponge from the collections of the State Museum of Pennsylvania provides potential evidence of commensalism, with the crinoid benefitting from the sponge’s feeding currents. Potentially, the association also offered the crinoid protection during a time of increasing predation intensity. The fossil was collected in the 1800s from a locality near the town of Warren, likely from the lower Knapp Formation. Caster (1939, Journal of Paleontology, 13:1-20) illustrated an ophiuroid preserved inside a hexactinellid from a nearby location, so echinoderm-hexactinellid commensalisms may have been common, though rarely preserved.

Second, funnel-shaped trace fossils from marginal marine strata of the Catskill Formation in Tioga County occur in unusual gregarious clusters that suggest intraspecific coordination among the trace-makers. The burrows are preserved as casts on the bases of beds, in full relief within beds, and as filled burrow openings on the tops of beds. They are assigned to the ichnotaxon Lingulichnus verticalis, which is commonly made by linguilide brachiopods; lingulides occur in these strata, although they have not been found within the burrows. In some cases, the burrows are organized in linear chains along bedding planes, with adjacent burrows touching or nearly touching each other but not cross-cutting. Burrows are often stacked as a result of equilibration to sediment deposition, and clusters of trace-makers maintained their linear associations while burrowing upwards following sedimentation. By analogy with modern examples, the linear arrangement 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.