LEARNING TO DRILL IN THE WIS: EXAMPLES FROM THE FOX HILLS FORMATION

The record of predatory gastropod drillholes seen in the molluscan faunas of the Western Interior Seaway (WIS) provides an independent test of many aspects of the "Escalation Hypothesis". Kase and Ishikawa’s (2003) intriguing conclusion that true (drilling) naticids only evolved in the Late Campanian appears to be independently supported by the record of WIS drilling. Prior to the latest Campanian, only a few suspect drillholes are documented from the faunas of the entire WIS. Following this, naticid taxa and drillholes become more prevalent within the WIS. In the well-preserved and diverse Maastrichtian Fox Hills fauna, drilling intensity reaches 2.1%. This drilling intensity is low compared to the better-known patterns obtained from the coeval U. S. Coastal Plain. This suggests that the Coastal Plain pattern is not representative of all Cretaceous localities, and this early phase of naticid predation in the WIS also offers interesting contrasts to predator-prey dynamics in the Cenozoic. Despite the low overall predation intensity, the bivalve prey in the WIS represent a broader range of taxonomic groups, morphologies, and life/trophic habits than has been documented in bivalve prey taxa in contemporaneous Coastal Plain faunas. These patterns are suggestive of a relatively low degree of prey specificity on the part of the WIS predator. Secondly, there appears to be little relationship between the abundance of individuals within a prey taxon and the frequency with which they were drilled, again in contrast to the pattern observed in the Coastal Plain and more recent deposits for which a strong correlation between rank abundance and drilling intensity has been documented. Thirdly, Fox Hills naticids are typically found in much greater frequency than in later deposits, such as the Plio-Pleistocene of Florida, although the drilling intensity is significantly lower. This suggests that drilling was not occurring as frequently as might be predicted from more recent examples, possibly because additional modes of predation such as smothering were more prevalent, or these earlier naticids were not strictly carnivores. The various features of the drilling record in The Fox Hills Formation suggest a level of fluidity in prey selectivity and drilling predation in naticids that no longer exists.