Psilonichnus lutimuratus n. ichnosp. is described from a Pliocene estuarine-mouth depositional environment (Skolithos ichnofacies) of the Olympic Peninsula, Washington, U.S.A. These simple Y-, I-, and J-shaped, mud-lined burrows occur in situ as dense patches within alternating, wavy-bedded sandstone and mudstone in a storm and flood influenced coastal sequence from an active tectonic margin. The I- and J-shaped traces represent erosional modification of burrow tops during storm-flood events. The new ichnospecies differs from the two other Psilonichnus ichnospecies by the distinct mud-lining of the burrow wall. Comparison with living thalassinoidean shrimp burrows and shrimp ecology allow this new ichnospecies to be attributed to the extant mud shrimp Upogebia. Biological and behavioral characteristics of this living shrimp restrict it to the mouth of the open estuary, and these parameters can be used to narrowly define a shoreline environment in the stratigraphic record.

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