Biomechanical analyses of pterygotid sea scorpion chelicerae uncover predatory specialisation within eurypterids

Abstract

Eurypterids (sea scorpions) are extinct aquatic chelicerates. Within this group, members of Pterygotidae represent some of the largest known marine arthropods. Representatives of this family all have hypertrophied, anteriorly-directed chelicerae and are commonly considered Silurian and Devonian apex predators. Despite a long history of research interest in these appendages, pterygotids have been subject to limited biomechanical investigation. Here, we present finite element analysis (FEA) models of four different pterygotid chelicerae—those of Acutiramus bohemicus, Erettopterus bilobus, Jaekelopterus rhenaniae, and Pterygotus anglicus—informed through muscle data and finite element models (FEMs) of chelae from 16 extant scorpion taxa. We find that Er. bilobus and Pt. anglicus have comparable stress patterns to modern scorpions, suggesting a generalised diet that probably included other eurypterids and, in the Devonian species, armoured fishes, as indicated by co-occurring fauna. Acutiramus bohemicus is markedly different, with the stress being concentrated in the proximal free ramus and the serrated denticles. This indicates a morphology better suited for targeting softer prey. Jaekelopterus rhenaniae exhibits much lower stress across the entire model. This, combined with an extremely large body size, suggests that the species likely fed on larger and harder prey, including heavily armoured fishes. The range of cheliceral morphologies and stress patterns within Pterygotidae demonstrate that members of this family had variable diets, with only the most derived species likely to feed on armoured prey, such as placoderms. Indeed, increased sizes of these forms throughout the mid-Palaeozoic may represent an ‘arms race’ between eurypterids and armoured fishes, with Devonian pterygotids adapting to the rapid diversification of placoderms.

Ex Vivo Three-Dimensional Reconstruction of Acutiramus: A Giant Pterygotid Sea Scorpion

“…Pterygotid chelicerae were undoubtedly used to capture prey for processing by coxal gnathobases (Waterston, 1964; Miller, 2007; Bicknell et al, 2022b). This predatory ecology is further supported through correlations between chelicerae and lateral compound eye shapes (Anderson et al, 2014; McCoy et al, 2015) and 3D biomechanical analyses (Bicknell et al, 2022b).…”

“…Pterygotid chelicerae were undoubtedly used to capture prey for processing by coxal gnathobases (Waterston, 1964; Miller, 2007; Bicknell et al, 2022b). This predatory ecology is further supported through correlations between chelicerae and lateral compound eye shapes (Anderson et al, 2014; McCoy et al, 2015) and 3D biomechanical analyses (Bicknell et al, 2022b). For Acutiramus specifically, species of this genus have serrated, oblique, elongated, anteriorly directed, denticles (Chlupáč, 1994; Laub et al, 2010; McCoy et al, 2015; Bicknell et al, 2020).…”

“…Additional specimens from Lang's Fossils (Ilion, NY) were also considered. Finally, we employed published high-resolution images of Acutiramus and other pterygotids (Waterston, 1964; Chlupáč, 1994; Tetlie and Briggs, 2009; Laub et al, 2010; Bicknell et al, 2022b). Fossils that illustrate key aspects of the morphology are shown in figures 2–7.…”

“…One Paleozoic arthropod group worth considering with ex vivo reconstruction the sea scorpions (eurypterids). Recent ex vivo 3D reconstructions of eurypterids have focused on appendages to understand range of motion (Bicknell et al, 2022c; Schmidt et al, 2022) and cheliceral biomechanics (Bicknell et al, 2022b). These reconstructions have been necessary as most eurypterids are preserved as near 2D impressions (although see examples of cuticular preservation in Holm, 1898, Selden, 1981, Lamsdell et al, 2015, and Bicknell et al, 2018b) and rare 3D preservation (Lamsdell et al, 2020).…”

“…These structures are far larger than those in any other eurypterid, even the closely related Slimonia Page, 1856 (Kjellesvig-Waering, 1964; Waterston, 1964). As such, there has been considerable research into these structures (see Kjellesvig-Waering, 1964; Waterston, 1964; Bicknell et al, 2022b), with the overall conclusion that they are used in predation.…”