CRYPTIC TRACE FOSSILS AS CLUES TO THE TAPHONOMY OF BACULITID CONCHS (UPPER CRETACEOUS PIERRE SHALE, WESTERN INTERIOR, NORTH AMERICA)

The straight ammonite Baculites is abundant in parts of the Pierre Shale (Upper Cretaceous) in the Western Interior of North America. The most common deposits consist of thousands of internal molds, many with remnants of original aragonite shells, in dark shales. These internal molds are concretionary, usually filling the conchs with very fine-grained sediment. The prominent questions here include how much time the conchs spent in the water column after death, how long they sat on the seafloor before burial, and what diagenetic processes caused them to be filled with sediment. Trace fossils found in the conch interiors provide important clues about the taphonomy of these baculitids, and they reveal a cryptic community living on the Pierre seafloor which is otherwise poorly known. Two ichnospecies of Radulichnus are preserved as concave epireliefs on shell interiors and convex hyporeliefs on the exteriors of baculitid internal molds from Pierre exposures in South Dakota and Wyoming. One has arcuate, parallel striae consistent with the radular scrapings of a polyplacophoran (chiton). Another is apparently a product of a docoglossan radula such as is found in patellogastropods (true limpets). A similar surficial trace fossil present is a set of arcuate striae that overlap in a cross-hatched pattern suggesting the systematic scratchings of a crustacean grazing the interior surface of the baculitid conch. Three varieties of the firmground trace fossil Arachnostega, distinguished by tunnel diameter and branching patterns, are common on the baculitid internal molds. Taken together with encruster evidence, the trace fossils support a taphonomic model in which the baculitid conchs floated for some period after death and then dropped to the seafloor where organic material was scraped from the shell interiors by polyplacophorans, limpet gastropods, and crustaceans. Sediment filled the shells upon burial and was mined by vermiform deposit-feeders before full cementation.