A Trilobite Cluster from the Silurian Rochester Shale of New York: Predation Patterns and Possible Defensive Behavior

Evidence of predator-prey interactions in the fossil record offers important insights into extinct ecosystems. As direct predator-prey relationships are rarely preserved, records of failed predation upon prey species are often considered. The biomineralized exoskeleton of trilobites is exemplary for recording injuries that have resulted from predation. Despite the extensive documentation of trilobite injuries, abnormal specimens are often documented in isolation, with examples of injuries among clustered individuals being poorly known. Here we document a well-preserved body cluster of 18 individuals of the large lichid trilobite Arctinurus boltoni from the mid-Silurian (Wenlock) Rochester Shale of New York, with eight specimens showing injuries. Landmark geometric morphometrics of the specimens is used to explore possible patterns between injured and noninjured specimens. Results of the morphometric analysis indicate that injured and noninjured specimens do not show any systematic difference in overall shape of the exoskeleton, but many of the larger specimens have injuries. The majority of injuries are posteriorly located and right-side dominant, highlighting the possibility of predator or prey lateralization. Biostratinomic evidence suggests that the cluster represents a biological aggregation that was rapidly buried in situ. Potential reasons for this gregarious behavior are discussed, including the possibility that individuals of A. boltoni grouped together to provide “safety in numbers” against predatory attack.