Reappraisal of the Silurian placoderm Silurolepis and insights into the dermal neck joint evolution

Silurolepis platydorsalis, a Silurian jawed vertebrate originally identified as an antiarch, is here redescribed as a maxillate placoderm close to Qilinyu and is anteroposteriorly reversed as opposed to the original description. The cuboid trunk shield possesses three longitudinal cristae, obstanic grooves on the trunk shield and three median dorsal plates, all uniquely shared with Qilinyu. Further preparation reveals the morphology of the dermal neck joint, with slot-shaped articular fossae on the trunk shield similar to Qilinyu and antiarchs. However, new tomographic data reveal that Qilinyu uniquely bears a dual articulation between the skull roof and trunk shield, which does not fit into the traditional ‘ginglymoid’ and ‘reverse ginglymoid’ categories. An extended comparison in early jawed vertebrates confirms that a sliding-type dermal neck joint is widely distributed and other types are elaborated in different lineages by developing various laminae. Nine new characters related to the dermal neck joint are proposed for a new phylogenetic analysis, in which Silurolepis forms a clade with Qilinyu. The current phylogenetic framework conflicts with the parsimonious evolution of dermal neck joints in suggesting that the shared trunk shield characters between antiarchs and Qilinyu are independently acquired, and the sliding-type joint in Entelognathus is reversely evolved from the dual articulation in Qilinyu.