Abstract
BACKGROUND: Euarthropod appendages are specialized for diverse roles including feeding, walking, and mating, which require precise morphologies and ranges of motion. Cambrian fossils preserve exceptional details of extinct euarthropod appendages that can illuminate their anatomy and ecology. However, fossils are typically restricted by small sample sizes or incomplete preservation, and thus functional studies of the appendages usually rely on idealized reconstructions. The Burgess Shale Olenoides serratus is unique among trilobites owing to the availability of numerous specimens with soft tissue preservation that allow us to quantify its appendages' functional morphology. RESULTS: We measured the range of motion of the legs in Olenoides serratus and the extant horseshoe crab Limulus polyphemus. Despite repeated ecological comparisons between trilobites and xiphosurans, we find significant differences in the appendages' mobility between these taxa, with Limulus showing greater flexibility between the podomeres. CONCLUSIONS: O. serratus legs have a more restricted range of motion relative to L. polyphemus, particularly in their distal region. Flexure between the protopodite played a critical role in allowing the endopodite to create known trilobite trace fossils and bring food toward the ventral groove.