PARADOX OF DRILLED DEVIL'S TOENAILS: TAPHONOMIC MIXING OBSCURES CRETACEOUS DRILLING PREDATION IN UTAH OYSTERS

Abstract

ABSTRACT Pycnodonte newberryi oysters accumulated in voluminous shell beds of Late Cretaceous (late Cenomanian) age in shale deposits of the Western Interior Seaway. Analysis of triplicate grid samples from six shell beds in southern Utah shows that the reclining valves of the oyster are disproportionately represented by the inflated left valve, eight times more often than the smaller, flat right valve, and whole valves are rare. The shells nearly always fragment from the commissure toward the thick umbonal half of the shell. These fragmented shells make up more than half of grid samples, and only 0.5% contain predatory drillholes. By contrast, complete shells are drilled nearly 5% of the time, particularly near the umbo. Fragmentation cannot account for this discrepancy because the preponderance of drillholes occurs in the umbonal part of the shell, the part that is always preserved. To understand this paradox, drilled Pycnodonte were collected from a separate site in Utah and drillhole position on the s...