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

Abstract

Pterygotid eurypterids include some of the largest aquatic arthropods in the fossil record and are known from middle Silurian to Middle Devonian deposits across the globe. These forms primarily preserve as mostly two-dimensional impressions, a situation that has impacted the accurate reconstruction of pterygotid ventral structures and organization. By documenting specimens displaying important dorsal and ventral structures from both Acutiramus-a larger, well-documented pterygotid genus-and other pterygotids, we reconstruct Acutiramus in three-dimensions (3D) to more thoroughly understand the functional morphology and limb arrangement of these large eurypterids. With this comprehensive 3D reconstruction, we demonstrate a much more anterior insertion of appendages II-V, a nearhorizontal orientation of appendages II-VI coxae, the presence of reduced appendage II, and a labrum and epistomel region. The labrum and epistomel sections are identified as the locality for cheliceral articulation. This model also uncovers the streamlined nature and lowprofile morphology of Acutiramus. We use our model to explore the morphofunctionality of pterygotid eurypterids, including their feeding strategies, addressing a conundrum inherent in previous pterygotid reconstructions.

The “elongate chelicera problem”: A virtual approach in an extinct pterygotid sea scorpion from a 3D kinematic point of view

“…Additionally, we incorporated recent findings on appendicular insertion and orientation, as well as the reduced appendage II, from Bicknell, Kenny, and Plotnick (2023). Although mistakenly Bicknell, Kenny, and Plotnick (2023) introduced a more robust, thicker, and longer Eurypterus ‐like “swimming leg” (appendage VI) to their reconstruction of the pterygotid Acutiramus . In contrast, the Pterygotidae exhibit slender and smaller appendages (Bicknell, Kenny, & Plotnick, 2023, figure 8).…”

“…The true nature of the chelicerae, whether euthygnathous or klinognathous, remains unknown. In nearly all documented pterygotid specimens in the literature, the chelicerae are preserved disarticulated from the body (Bicknell, Kenny, & Plotnick, 2023;Clarke & Ruedemann, 1912;Kjellesvig-Waering, 1964;Laub et al, 2010;Størmer, 1955). This may be explained by molts, as molting in sea scorpions could have presented several challenges, particularly with the elongated chelicerae in pterygotids, which would pose a particular difficulty (Brandt, 2021;Tetlie et al, 2008).…”

“…The model integrates informative morphological data from the original description of Pterygotus buffaloensis (referred to as Acutiramus cummingsi (Grote & Pitt, 1877), as depicted in Clarke &Ruedemann, 1912, andsubsequently in Størmer, 1955). Additionally, we incorporated recent findings on appendicular insertion and orientation, as well as the reduced appendage II, from Bicknell, Kenny, and Plotnick (2023). Although mistakenly Bicknell, Kenny, and Plotnick (2023) introduced a more robust, thicker, and longer Eurypterus-like "swimming leg" (appendage VI) to their reconstruction of the pterygotid Acutiramus.…”

“…Upon reanalysis of the same material (BMS E837), Bicknell, Kenny, and Plotnick (2023) demonstrated that the structure was more likely a crack than a joint, challenging the interpretation of Kjellesvig‐Waering (1964) of four elements. Consequently, the current understanding of Pterygotida, specifically Acutiramus , posits that their chelicerae comprised only three elements.…”

“…Consequently, the elongated chelicerae of Pterygotida were initially thought to consist of four elements (Clarke & Ruedemann, 1912; Kjellesvig‐Waering, 1964), a notion later revised to three (Bicknell, Kenny, & Plotnick, 2023; Laub et al., 2010). Laub et al.…”