The paleoenvironmental significance of Psilonichnus

The ichnogenus Psilonichnus and its named ichnospecies are evaluated for their utility in paleoenvironmental reconstructions. A combined ichnological-sedimentological model for shoreface-estuarine-fluvial settings is presented herein to show distributions of the ichnospecies of Psilonichnus in an ichnofacies framework. High-resolution differentiation of coastal paleoenvironments may be achieved in strata containing Psilonichnus only if the trace fossils are identified to ichnospecies level, and if they are assessed as sedimentary structures within a depositional context. The simple Y-, J-, and I-shaped, vertical to inclined burrows referred to as Psilonichnus include three distinct ichnospecies- P. tubiformis, P. upsilon, and P. lutimuratus. Each can be attributed to either of two decapod crustacean progenitors, thalassinoid mud shrimp, or ocypodid ghost crabs. The ichnogenus occurs in association with the Psilonichnus, Glossifungites, and Skolithos ichnofacies in estuary / bay to backshore / dune sedimentary settings. Psilonichnus has proved to be an effective paleoenvironmental indicator, especially in studies of sequence stratigraphy. To date, however, inconsistencies in both ichnotaxonomic and ichnofacies assignments for Psilonichnus, and confusion regarding probable tracemakers, have diminished its usefulness to sedimentary geology. To avoid such problems, each new field occurrence of a potential Psilonichnus trace must be evaluated thoroughly within its biological, ecological, and physical context.