PARADOX OF THE DEVIL’S TOENAILS: TAPHONOMIC MIXING OBSCURES CRETACEOUS DRILLING PREDATION IN UTAH OYSTERS

Pycnodonte newberryi oysters, commonly known as Devil’s Toenails, accumulated in voluminous shell beds of Cenomanian age in shale deposits of the Western Interior Seaway. Analysis of triplicate bulk samples from six shell beds 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), and whole valves are rare. Fragmentation of the shells nearly always erodes from the commissure to the thick umbonal half of the shell; these fragmented shells comprise the majority (59%) of bulk samples, and only 0.5% contain Oichnus predatory drillholes. By contrast, complete shells are drilled nearly 5% of the time and especially near the umbo. Fragmentation cannot account for this discrepancy because the preponderance of drillholes evidently occurs in the part of the shell that is preserved.

To understand this paradox, a new sample of drilled Pycnodonte (n = 91) was collected from a separate site in Utah and drillhole position on the shell was mapped to create a probabilistic model of drillhole site. Results show that >60% of drillholes are positioned in the robust part of the umbonal shell, confirming observations in the unfragmented bulk-sampled shells. We propose that the deficit in drilled shell fragments can be explained by biostratinomic mixing of two populations of Pycnodonte that lived under separate predation pressures. Autochthonous shells show greater drillhole incidence and shell completeness, whereas fragmented umbonal shells indicate transportation from a separate, perhaps salinity-stressed environment, having lower predation pressure. This study points to taphonomic mixing, rather than fragmentation, as the driving bias in drilling incidence.