A neoichnological resin cast of modern burrows from a mudflat at Willapa Bay, Washington, reveals much of the architecture of the infaunal community present locally within sediments of this mudflat. The cast structures comprise Psilonichnus-, Skolithos-, Trichichnus-, Chondrites(?)-, Planolites-, Palaeophycus-, and Arenicolites-like burrows. The range of behaviors represented by these structures cannot be classified into an archetypal (Seilacherian) ichnofacies.

The resin cast contains many coeval, composite burrows that may reflect certain commensal relationships. Notable composite associations include: (1) siphon shafts of Mya arenaria that are connected to and descend from crab domiciles; (2) an example of a pair of infaunal Mya arenaria that share a single siphonate shaft; (3) abundant Heteromastus burrows that connect to crab and bivalve burrows; and, (4) Nereis burrows that are connected to crab domiciles. It is suggested that such contemporaneous relationships, if passed into the fossil record, might be misinterpreted as representing tiering or faunal succession. The reported observations provide a new perspective on the potential significance of interpenetrating trace fossils and complex ichnofabrics.

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