Temporal patterns in successful and unsuccessful shell-breaking predatory attack strategies on Varicorbula in the Plio-Pleistocene of Florida

• Traces of successful (breaks) and unsuccessful (scars) attacks are studied in tandem.
• Traces are classified according to their sizes and locations on prey shells.
• When used independently, scars and breaks have limited interpretative power.
• In tandem, scars and breaks provide information regarding relative predator success.

Shell-breaking predators can employ multiple strategies when they attack their molluscan prey. Some of these attacks are highly effective and lead to the successful subjugation of prey and should be favored by natural selection, whereas other attacks are more likely to fail. Here, we use an analysis combining data from both the size and position of successful (unrepaired shell breaks) and failed attacks (repair scars) by shell-breaking predators on Varicorbula spp. to examine how the effectiveness of different attack strategies has changed through the Plio-Pleistocene of Florida. When data from these two types of attacks are studied in tandem, it suggests that predators, most likely crabs, preferentially attacked (both successfully and unsuccessfully) the posterior portions of the shell during the Plio-Pleistocene Pinecrest Beds and the latest Pleistocene Ft. Thompson, whereas during early Pleistocene Caloosahatchee and middle Pleistocene Bermont, they favored the ventral region of the shell. In terms of predatory success relative to the site and the length of shell damage, extensive breaks were almost always successful, and minor to medium attacks are the most common type of attack. Furthermore, the broader implications of this study suggest that interpretations based either on scars or breaks in isolation as an indicator of predation can lead to erroneous interpretations, and breaks and scars in association with size and location information should be used in tandem to better constrain potential ecological interpretations.