Two types of ichnofossils from Pleistocene outcrop at Willapa Bay are described. Because both trace fossils are characterized 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 upsilo. Type A and B.
Psilonichnus upsilo. Type A is generally 1 to 3 cm in diameter and is infilled with laminated sediment. In general, P. upsilo. Type A is observed in ancient point-bar deposits. It has an extremely simple architecture that is almost identical to that produced by the crab Hemigrapsus oregonensi. in modern tidal flats at Willapa Bay. Psilonichnus upsilo. Type B normally exceeds 10 cm diameter and is infilled with laminated sediment. The passive infill commonly is deposited in couplets and may be delivered to the burrow network by tide-generated currents. Psilonichnus upsilo. 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 upsilo. has a more variable architecture than discussed in the literature. The size and angle of the tunnel are variable, and Psilonichnu. may aggrade, forming Teichichnu.-like structures. (2) In the modern bay, burrowing shrimp dominate subtidal, point-bar, and intertidal deposits. 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. upsilo. Type B may not be present in the modern bay. However, such a trace maker was present when these Pleistocene deposits accumulated.