“Whether the upper limb branch of Paleozoic “biramous” arthropods, including trilobites, served a respiratory function has been much debated. Here, new imaging of the trilobite Triarthrus eatoni shows that dumbbell-shaped filaments in the upper limb branch are morphologically comparable with gill structures in crustaceans that aerate the hemolymph. In Olenoides serratus, the upper limb’s partial articulation to the body via an extended arthrodial membrane is morphologically comparable to the junction of the respiratory book gill of Limulus and differentiates it from the typically robust exopod junction in Chelicerata or Crustacea. Apparently limited mechanical rotation of the upper branch may have protected the respiratory structures. Partial attachment of the upper branch to the body wall may represent an intermediate state in the evolution of limb branch fusion between dorsal attachment to the body wall, as in Radiodonta, and ventral fusion to the limb base, as in extant Euarthropoda.”