A FOSSIL ASSEMBLAGE OF CASSID GASTROPODS AND WHALE BONES: TAPHONOMY AND PALEOECOLOGICAL INTERPRETATION

Numerous, large-shelled cassid gastropods are preserved in Oligocene mudstones, closely associated with fossil mysticetes. Cetacean skeletons and a highly diverse molluscan and crustacean fauna are well preserved in the organic-rich sediments of the Pysht Formation (Twin Rivers Group), of the northern Olympic Peninsula, Washington. Disarticulated mysticete skeletons are preserved in calcite-cemented concretions, with teredo-bored wood and fine particles of unidentifiable plant material. The cassid gastropods, Liracassis rex, were found preserved within the concretions, and many of them are oriented so that the aperture was in direct contact with the bone surface. Living gastropods and bivalves found associated with bones of whale carcasses have been reported from +1000 m depths in the eastern Pacific, and the trophic system of these isolated ecological units has been attributed to a chemoautotrophic bacteria-based system. However, evidence here points to the spatial distribution of the gastropods on bone areas devoid of bacterial mats. The distribution of living cassid gastropods of the northern Pacific are restricted to latitudes south 33° N along the eastern margin, and south of 35° N on the western margin. This taxon is characterized as feeding exclusively on echinoids by actively boring into living prey items. The taphonomy of the Oligocene mysticete skeletons and associated mollusks indicates that the cassids were scavenging whale flesh or bone. Other invertebrates found within the bone-bearing concretions were rare solemyid bivalves, naticid gastropods, balanid barnacles plates, and shark teeth. Although brissid echinoids are present in this formation, no plates or spine occurred in the concretions. In the horizontal surface of the outcrop, whale bones were scattered and the smaller bones of the forelimb and tail missing. Taphonomic signatures indicate that the bones and invertebrates we not transported together by bottom current into a topographic low. Post-depositional scavenging and soft tissue decay occurred prior to burial.