FIRST EVIDENCE OF DRILLING PREDATION IN INOCERAMIDS

The history of drilling predation continues to be the focus of considerable research largely fueled by the evolutionary implications of the "arms’ race" as cogently formulated in numerous papers by Vermeij. Here, we document two inoceramids from the Western Interior Seaway (WIS) that exhibit drillholes; the first evidence of drilling predation discovered in this bivalve group. It should be noted that although this is this drilling is novel, other types of predation as well as other types of non-predatory drilling have been documented in this group. The two drilled specimens were collected from a single, inoceramid-rich, limestone concretion at the Texas Trail locality northeast of Moorcroft, WY and come from the Baculites eliasi biozone in the upper unnamed member of the Pierre Shale. Based on the drillhole morphology, as well as paleoecologic evidence, these features are attributed to naticid gastropods. One of the intriguing questions long associated with inoceramids is the virtual absence of any record of drilling predation, despite the group’s long evolutionary history and ecological dominance of Late Mesozoic, especially Late Cretaceous, marine deposits. To address the paucity of drilling evidence in inoceramids, we explore several potential explanations, including: 1) non-overlapping environmental and temporal ranges, 2) the unique structure of inoceramid shell; and 3) the overall history of drilling predation in the WIS. The faunal data suggests that prior to the latest Campanian inoceramids and naticids favored different habitats. However, commencing in B. eliasi time, the two are often found in the same concretions, suggesting that their habitat preferences converged. Drilling predators may also have been deterred by the inoceramids prismato-nacreous shell. This unusual microstructure, exhibited by bivalves such as Pinna today, results in a flexible shell that may have been resistant to gastropod drilling. Finally, taken in its entirety, the record of predation in the WIS displays not only a delayed initiation of drilling but also a lower intensity when compared to documented global patterns. This may explain, in part, the paucity of drilled inoceramids in the WIS, although given their global distribution it fails to explain the absence of drilled specimens from other regions.