Sea scorpions (Eurypterida; Euchelicerata) explored the extreme limits of the aquatic euchelicerate body plan. Indeed, the group contains the largest known marine euarthropods. Inferences on eurypterid life modes—in particular walking and eating—are commonly made by comparing the group to horseshoe crabs (Xiphosura; Euchelicerata). However, no models have been presented to test these hypotheses. Here, we reconstruct prosomal appendages of two exceptionally well-preserved eurypterids Eurypterus tetragonophthalmus and Pentecopterus decorahensis and kinematically model the flexure and extension of these appendages in 3D. We compare these models to 3D kinematic models of Limulus polyphemus prosomal appendages. This comparison highlights that the examined eurypterid prosomal appendages could not have effectively moved prey items to the gnathal edges, and therefore would not have emulated the motion of a L. polyphemus walking leg. It seems that these eurypterid appendages were primarily used to walk or grab prey and other appendages would have moved prey for mastication. Such 3D kinematic modelling highlights how eurypterid appendage morphologies placed substantial limits on their function, suggesting a high degree of specialisation, especially when compared to horseshoe crabs. 3D kinematic modelling of these extinct groups also presents an innovative approach to understanding the position of these animals within their respective paleoecosystems.