VESTIGES OF MARINE VERTEBRATE FEEDING BEHAVIOR ON DEEP-BURROWING BIVALVES: THEIR PALEOENVIRONMENTAL SIGNIFICANCE

Quarrying of coarse-grained Pleistocene glaciomarine deposits adjacent to Willapa Bay, Washington, has recently exposed a contact from which unusual sedimentary structures originate. These structures manifest two distinct occurrences: (i) vertical to sub-vertical columns where laminae and bedding deflect downward; and (ii) symmetric or asymmetric u-shaped structures with flared limbs containing occurrences of graded bedding. Considerations of scale, morphological complexity, and stratigraphic distribution defy the contention that physical sedimentary processes generated the structures. Rather, they are more akin to biogenic sedimentary structures within the sea floor generated by the predatory action of marine animals on deep-burrowing bivalves. Several animals have been reported to predate on bivalves including: elasmobranch fishes, crabs, sea stars, sea otters, whales, and walruses. In particular, walruses generate distinctive excavations on the sea floor as they root for prey with their snouts and emit a hydraulic jet that liquefies the substrate where a bivalve has burrowed. The prey is subsequently extracted from the sediment slurry by the walrus and the soft parts are suctioned from the valves and consumed. Documentation in recent years of sea floor furrows and pits on the Bering Shelf and Chukchi Sea produced by the Pacific walrus (Odobenus rosmarus divergens) provides modern analogues for comparison to the ancient trace fossils described from Willapa Bay. Significant implications drawn from this comparison include: (i) bivalve mollusks have remained a staple of walrus diet since at least the Pleistocene; (ii) the method of hydraulic jetting employed by walruses for consumption of their prey has also remained constant; (iii) these predation structures are temporally significant in that they provide a minimum time of exposure and corresponding rate of accretion for the ancient estuary inlet; (iv) feeding excavations in paleo-Willapa Bay, Washington, were produced by walrus herds that wandered from the northern Pacific ice front during the Pleistocene after becoming barricaded from their present habitat in the Bering Shelf and Chukchi Sea.