New trace fossil evidence for eurypterid swimming behaviour

Abstract

We describe a recently discovered trace fossil from a eurypterid Konservat‐Lagerstätte in the upper Silurian Tonoloway Formation of Pennsylvania, and formally describe contemporaneous traces from the Williamsville Formation Lagerstätte of Ontario. The traces from both localities are assigned here to Arcuites bertiensis igen. et isp. nov. Based on comparisons with previously described eurypterid trackways, neoichnological experiments, and the co‐occurrence 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 unambiguous trace fossil evidence for eurypterid swimming behaviour. The morphology of Arcuites indicates that eurypteroid eurypterids swam using drag‐based rowing, whereby the animal propelled itself forward by moving its oar blade‐like swimming paddles in an in‐phase backstroke. Arcuites morphology also indicates that the eurypteroid swimming appendage had a greater degree of movement than was previously suggested, and a revised rowing model is proposed. Differences in the abundance of A. bertiensis in the Tonoloway and Williamsville formations suggest a bathymetric control on eurypterid swimming behaviour and trace production. The association of Arcuites with eurypterid body fossils in both units indicates that these Lagerstätten were autochthonous assemblages and provides additional evidence for eurypterid inhabitation of shallow subtidal marine environments in the late Silurian.

A eurypterid trackway from the Middle Ordovician of New York State

“…Palmichnium culmicum Pfeiffer, 1968 lacks tracks and therefore should be excluded. Arcuites bertiensis Vrazo and Ciurca, 2017, a eurypterid swimming trace from the Silurian of Ontario and Pennsylvania, looks like undertracks of Palmichnium , but the direction of travel is opposite to that interpreted by these authors.…”

A review of the glacial environment arthropod trace fossils Umfolozia and Warvichnium with the description of new ichnotaxa

“…The comparison with Warvichnium ulbrichi is given in the remarks on Warvichnium . Arcuites Vrazo & Ciurca, 2018 (Fig. 2T) is morphologically similar to Irichnus , with a series of arch‐shaped imprints but differs from Irichnus by having paired arches, either side of the midline of the trace, in opposite symmetry that have a curvilinear or lunate shape.…”

“…Petalichnus capensis was interpreted as being produced by eurypterids (Braddy & Almond 1999), with the internal impressions attributed to the dragging of the telson; however, the internal traces could be produced by an analogous anatomical structure also present in the U. terere tracemaker. The trackway Arcuites bertiensis is attributed to eurypterids when swimming (Vrazo & Ciurca 2018), and it resembles Irichnus due to the serial arches, but they are simpler, without scratches, and A. bertiensis has a medial impression produced by the telson, and larger series length (Vrazo & Ciurca 2018). Arcuites bertiensis and Irichnus were probably produced by analogue behaviours, but the morphological differences make an eurypterid tracemaker for Irichnus less probable, and there are few records of eurypterids in the Carboniferous of the southern hemisphere (e.g.…”

“…The imprints also have a much thinner width than Irichnus ; and in some ichnospecies it has a medial linear impression, which is absent in Irichnus . Nevertheless, Arcuites may be a behavioural analogue (swimming) (Vrazo & Ciurca 2018) of Irichnus . Arborichnus Romano & Meléndez, 1985 (Fig.…”