Gymnotrachelus hydei is a large selenosteid arthrodire placoderm known from the Late Famennian Cleveland Shales of Ohio. It was originally reconstructed as physically resembling Selenosteus, with slightly smaller orbits (i.e., having a broad, box-like head). Later specimens led to a reappraisal, and now G. hydei is thought to have a more gar-like or barracuda-like build.