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

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 near-horizontal 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 low-profile 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.