Reappraisal of the opercular anatomy in the ‘maxillate placoderms’ Entelognathus primordialis and Qilinyu rostrata

Abstract The Silurian ‘maxillate placoderms’ Entelognathus primordialis and Qilinyu rostrata were previously described as possessing a single dermal operculum, suspended by either an opercular cartilage or an antiarch-like dermal articulation, respectively. Here, we reinvestigate the holotypes of these taxa using synchrotron and micro-CT scanning data, revealing opercular morphologies distinct from prior descriptions. Instead of a single opercular plate as in other ‘placoderms’, Entelognathus possesses two dermal opercular plates arranged anteroposteriorly. The larger posterior plate bears a pronounced anterodorsal process, which was previously identified as the opercular cartilage. Both plates are extensively overlapped by the skull roof. The opercular morphology of Qilinyu, although consisting of a single opercular plate, shares features with that of Entelognathus, including a pronounced anterodorsal process and an extensive overlap by the skull roof. A comparison of the gill-covering structures in various early gnathostomes indicates that the described character combination is unique to the maxillate placoderms. The skull roof overlap suggests that the opercula of both taxa could slide along the visceral surface of the skull roof. Two new phylogenetic characters and several revised characters related to the operculum are proposed for an updated phylogenetic analysis of early jawed vertebrates. The Adams consensus tree from our analysis identifies phylogenetic signal for the monophyly of at least some of the maxillate placoderms.