SUBLETHAL INJURIES IN THE SCAPHITID AMMONOID HOPLOSCAPHITES NICOLLETII FROM THE LATE CRETACEOUS FOX HILLS FORMATION IN SOUTH DAKOTA (Invited Presentation)

Biotic interactions are an important driver for the evolution of organisms. Among those, predator-prey relationships play an essential role. In the Late Cretaceous of North America, ammonoids were one of the most abundant and diverse marine macroinvertebrates. Shell breakage on ammonoids has been reported as (sub)lethal injuries and often associated with predatory attacks. Despite the number of reports about such shell breakage, the temporal change in the frequency, position, and size through a stratigraphic succession is rare. Here, we examine ~1000 ammonoid specimens that bear shell breakage from the Upper Cretaceous Fox Hills Formation (Maastrichtian) in South Dakota, USA.

We examined four upper Maastrichtian ammonoid assemblage zones that include the lower nicolletii assemblage zone (LNAZ), the Limopis-Gervilla assemblage zone (LGAZ), the upper nicolletii assemblage zone (UNAZ), and the Protocardia-Oxytoma assemblage zone (POAZ). We classified the shell breakage into three types “ventral”, “lateral”, and “bilateral” breakages and investigated how the frequency, size, and position changed through time.

The breakage usually occurs at ~20–175º from the aperture, which falls within the part of the body chamber, and thus we assume that these marks represent (sub)lethal injuries. Additionally, the overall shape of the injuries is similar. Across the assemblage zones, we found about 10% of the specimens contain a type of breakage and there is no change through time. The “lateral” breakage was the most common type among the specimens at a frequency of about 5–10% and the “bilateral” breakage was the least common at about 3%. The ventral predation marks are the largest with maximum breakage of up to 52 mm representing 8% of maximum conch diameter and right and left lateral breakages are similarly sized. Considering the size, shape, and fossils previously reported from the Fox Hills Formation, we suggest Decapoda such as Cenomanocarcinus, Hoploparia, and/or Latheticocarcinus as possible predators.