A NEW BRANCHIOPOD CRUSTACEAN FROM THE MIDDLE DEVONIAN OF NEW YORK WITH IMPLICATIONS FOR THE EARLY EVOLUTION OF A KEY COMPONENT OF FRESHWATER COMMUNITIES (EUCRUSTACEA: BRANCHIOPODA)

The evolution of Branchiopoda (Eucrustacea) has received considerable attention as they have traditionally been viewed as resembling the ancestral crustacean, are adapted to freshwater environments, and recent molecular analyses have suggested a close relationship to Hexapoda (e.g., insects). While branchiopods have a rich fossil record, their early evolution remains poorly understood. Most described fossils with branchiopod affinities — other than the marine Cambrian Rehbachiella and the freshwater Devonian Lepidocaris — are derived members of crown Branchiopoda and provide limited insight into the origins of the group’s synapomorphies or its transition from marine to freshwater. We describe a putative branchiopod from the Middle Devonian Plattekill Formation of New York with morphology suggestive of an early divergence within the taxon. The fossils are preserved as carbonaceous cuticles in shale and represent an abundant component of a biota inhabiting an apparently lacustrine setting. The body of the branchiopod is divided into a well-defined head bearing a short head shield with a dorsal cervical groove, a thorax of 13 segments, and a limbless, six-segmented abdomen plus a telson terminating in two pairs of paddle-like caudal rami — a tail configuration unique to this taxon. Diagnosis as a branchiopod is founded on the presence of a few significant derived features: 1) remains of setal rows along the proximal inner edge of the thorax indicating a sternal food groove, 2) phyllopodous thoracopods with lobate setae-bearing endites, 3) mandibles with an expanded molar process comprising an ellipsoid plate rimmed by cusps. The thoracic tergites extend into distinct pleural lobes, a feature not known from Rehbachiella or recent branchiopods, but present in Lepidocaris. The new species resembles modern anostracans (fairy shrimps) and Lepidocaris, and may represent a stem member of Sarcostraca (Anostraca + Lepidocaris) that diverged prior to the reduction of the shield. However, many of the shared traits may be plesiomorphic for Branchiopoda, leaving open the possibility that it and possibly Lepidocaris are stem branchiopods. The new taxon provides a snapshot of the evolution of modern traits in Branchiopoda and highlights the deep history of branchiopods’ ecological role in freshwater habitats.