NEOICHNOLOGICAL NOVELTIES FROM SAPELO ISLAND (GEORGIA) AND THEIR APPLICATIONS TO THE FOSSIL RECORD

Sapelo Island is one of several Georgia barrier islands studied in the 1970s and 1980s by Robert W. Frey, James D. Howard, and many others for its modern organismal traces in salt marsh, foreshore, beach, and dune facies. These classic studies established one of the standard neoichnological models for marginal-marine facies used for interpreting trace fossil assemblages. Fortunately, new insights can still be added to this model, even from traces made by animals that are literally under the noses of beachgoers. Here we report new and recently documented traces made by two common species of tracemakers, the ghost crab (Ocypode quadrata) and horseshoe crab (Limulus polyphemus).

O. quadrata was observed recently in the upper intertidal zone of two Sapelo beaches making resting traces that are related to rehydration and respiration, rather than predator avoidance, the default hypothesis for resting traces. This physiologically based interpretation provides an alternative explanation for resting traces made by semi-terrestrial crabs or other arthropods. If recognized in the fossil record, such resting traces would provide finer distinctions between lower dune facies (Psilonichnus ichnofacies) and intertidal zone facies (Skolithos ichnofacies).

Juveniles of L. polyphemus were also observed making shallow burrows and surface trails on emergent intertidal sandflats and runnels on Sapelo; surprisingly, some of these traces closely resemble the trace fossil Nereites. Nereites historically was interpreted as a worm feeding trace in deep-water facies of post-Paleozoic strata and in shallow-water facies of Paleozoic strata. Our model upends that generality and is the only one based on a neoichnological example. Some limulid resting traces, like those of O. quadrata, are also linked to respiration, in which buried limulids move water across their gills and form collapse structures around their bodies. Other organisms, such as bivalves, echinoids, sea stars, mole crabs, and shorebirds are also reviewed as candidates for future neoichnological studies on Sapelo and other barrier islands of the Georgia coast.