DRILLING PREDATION OF OYSTER BEDS IN THE CRETACEOUS WESTERN INTERIOR SEAWAY, UTAH

Pycnodonte newberryi oyster shell beds are common in the uppermost Cenomanian (Upper Cretaceous) shales of the Western Interior Seaway. Analysis of triplicate bulk samples from six productive units in southern Utah show that the recumbent valves of the oyster are disproportionately represented by the inflated left valve (n = 4,292) over the smaller flat right valve (n = 499). Whole valves are rare. Post-mortem drillings of Entobia and Caulostrepsis affect nearly 25% of shells, further corroborating the time-averaged nature of the shell deposits. Predatory borings comprise a small, but ecologically important component of shell bioerosion. All predatory borings are circular, gently parabolic Oichnus. SEM imaging of epoxy casts show no glyph on the inside of the borings.

Oichnus borings affect, on average, up to 3% of Pycnodonte. Oichnus are concentrated in the thickest, dorsal part of the inflated left valve, and are absent from right valves of the bulk samples, although two were found after specifically searching for them in the field. Although right valves are taphonomically reduced in number, our sample size is sufficient to infer that right valve drilling was very rare, and that left valves were preferred targets. The recumbent lifestyle of Pycnodonte suggests that the predatory driller was infaunal, and/or manipulated the prey to drill into the left valve. Gastropods formerly assigned to the Naticidae (Eunaticina and Euspira) are contemporaneous potential predators in the Seaway, but neither are preserved in the nearly monotypic Pycnodonte beds.

It is unclear why only 3% of the copious potential biomass of Pycnodonte was consumed by drilling predators. Shell thickness, a recumbent lifestyle, and environmental isolation from predators do not appear to explain low drilling frequency. Instead, low predation appears to be persistent during the Cretaceous in the epeiric Western Interior Seaway, by contrast to elevated levels at the same time along the epicontinetnal Atlantic Coastal Plain.