EURYPTERID SWIMMING BEHAVIOR: INSIGHTS FROM NEW TRACE FOSSIL EVIDENCE

Eurypterids (Arthropoda: Chelicerata) were diverse and widespread aquatic Palaeozoic predators. Their predatory ability is thought to be due to a combination of size, visual acuity, and swimming aptitude. Whereas the first two traits can be assessed from fossil morphology, eurypterid swimming behaviour can only be determined through modeling or trace fossils. Trace fossil evidence for eurypterid walking or crawling behaviour, although rare, has been well-documented whilst unambiguous eurypterid swimming traces were heretofore unknown. Here, we describe a recently discovered trace fossil from a eurypterid Konservat-Lagerstätte in the upper Silurian of Pennsylvania that casts new light on contemporaneous traces from a eurypterid Lagerstätte in Ontario. The traces from both localities are considered to be related and are assigned to a new ichnogenus and ichnospecies, Arcuites bertiensis. Based on trace morphology and cooccurrence with eurypterid remains, Arcuites is interpreted as having been made by the swimming leg (sixth prosomal appendage) of swimming juvenile to adult eurypteroid eurypterids, and represents the first trace fossil evidence for eurypterid swimming behaviour. With respect to previously proposed eurypterid swimming models, Arcuites supports an in-phase rowing model in which the animal propelled itself forward by moving its oar blade-like swimming paddles in a synchronized backstroke motion. The association of Arcuites and eurypterid body fossils in the Tonoloway and Williamsville formations indicates that these are not transported assemblages and further constrains the eurypterids’ environmental preference.