Failed attack on the large Silurian trilobite Arctinurus suggests evidence of eurypterid predation

Records of failed predation provide important insights into predator-prey systems in the fossil record. Trilobites have been examined extensively in this context as failed attacks preferentially preserve on their biomineralized exoskeletons. Building on this framework, we present evidence of failed predation on a specimen of the Silurian (Wenlock) trilobite Arctinurus boltoni from the Rochester Shale. The specimen preserves four distinct indentations arranged in a semicircular pattern on the posterior thorax and pygidium. These features are interpreted as injuries produced by a predator while the trilobite was in a post-molt, soft-shelled state. The large chelicerae of pterygotid eurypterids are implicated as the producers of these injuries. This interpretation is supported quantitatively by comparable injury sizes and denticle widths, as well as distances between indentations that closely match spacing between primary denticles of chelicerae. This specimen represents possible evidence for interactions between these large arthropods and indicates that larger trilobites could escape predation by eurypterids. This highlights the role of body size in Silurian predator-prey interactions and possible size refugia within trilobites.