DRILLING INTENSITY IN THE LATE CRETACEOUS: FOX HILLS OR RIPLEY, BELIEVE IT OR NOT

A range of values has been reported for the intensity of drilling predation in the Late Cretaceous. Most of these values come from a single geologic unit, the Ripley Formation of the US Coastal Plain, and range in drilling frequency from 6 to 30%. These values have played an important role in documenting the history of drilling predation and the development of a general model of its evolution. Recently, they have been used to demarcate the onset of the "Cenozoic Phase", an interval of dramatically higher drilling frequency. Here we present evidence from a nearly contemporaneous unit, the Fox Hills Formation of the US Western Interior, which has a distinctly lower drilling intensity. Like the Ripley, the Fox Hills represents shallow-marine environments and contains a similar molluscan fauna. Furthermore, this unit is well exposed throughout northwestern South Dakota, and is renowned for its exceptional faunal preservation, making it an ideal candidate for this type of analysis. Based on an examination of approximately 5,000 specimens reflecting the entire bivalve fauna, the overall drilling frequency upon bivalves is slightly greater than 2%. This lower value is more typical of those reported from earlier in the Mesozoic (most Jurassic levels hover close to 2%) and thus appears anomalous when compared to the drilling frequencies reported from the Ripley Formation of the "Cenozoic Phase". These new data highlight the difficulty of deriving global patterns from spatially and stratigraphically restricted samples. They also beg larger questions about the intensity of Late Cretaceous drilling as well as the timing, setting, and taxonomic fabric of the onset of "Cenozoic Phase" levels of drilling predation.