The Significance of Pleistocene Psilonichnus at

Two types of ichnofossils from Pleistocene outcrop at Willa-pa Bay are described. Because both trace fossils are char-acterized by an inclined to horizontal tunnel, are unlined, have an exaggerated J-shaped morphology, rarely branch, and have an unconstricted apertural opening, they have been classified as Psilonichnus upsilon Type A and B. Psilonichnus upsilon Type A is generally 1 to 3 cm in di-ameter and is infilled with laminated sediment. In general, P. upsilon Type A is observed in ancient point-bar deposits. It has an extremely simple architecture that is almost iden-tical to that produced by the crab Hemigrapsus oregonen-sis in modern tidal flats at Willapa Bay. Psilonichnus up-silon Type B normally exceeds 10 cm diameter and is infil-led with laminated sediment. The passive infill commonly is deposited in couplets and may be delivered to the burrow network by tide-generated currents. Psilonichnus upsilon Type B is observed in intertidal flat deposits. The overall morphology of this trace fossil is most similar to burrows generated by large crustaceans such as crabs, stomatopods, and lobsters. The occurrence of these traces leads to four findings: (1) Psilonichnus upsilon has a more variable architecture than discussed in the literature. The size and angle of the tunnel are variable, and Psilonichnus may aggrade, forming Tei-chichnus-like structures. (2) In the modern bay, burrowing shrimp dominate subtidal, point-bar, and intertidal depos-its. The Pleistocene record indicates that burrowing crabs sometimes occupied similar niches in the ancient bay. (3) Laminated, heterolithic burrow fills provide evidence of rhythmic sedimentation. These laminae represent tidal or episodic sedimentation and provide the only evidence of such processes in otherwise muddy deposits. (4) A large burrowing crab that might make P. upsilon Type B may not be present in the modern bay. However, such a trace maker was present when these Pleistocene deposits accumulated