Redescription of Y inostius major ( A rthrodira: H eterostiidae) from the Lower D evonian of C hina, and the interrelationships of B rachythoraci

Y inosteus major is a heterostiid arthrodire (Placodermi) from the Lower Devonian Jiucheng Formation of Y unnan P rovince, south‐western C hina. A detailed redescription of this taxon reveals the morphology of neurocranium and visceral side of skull roof. Y inosteus major shows typical heterostiid characters such as anterodorsally positioned small orbits and rod‐like anterior lateral plates. Its neurocranium resembles those of advanced eubrachythoracids rather than basal brachythoracids, and provides new morphological aspects in heterostiids. Phylogenetic analysis based on parsimony was conducted using a revised and expanded data matrix. The analysis yields a novel scenario on the brachythoracid interrelationships, which assigns Heterostiidae (including H eterostius ingens and Y inosteus major ) as the sister group of D unkleosteus amblyodoratus . The resulting phylogenetic scenario suggests that eubrachythoracids underwent a rapid diversification during the Emsian, representing the placoderm response to the Devonian Nekton Revolution. The instability of the relationships between major eubrachythoracid clades might have a connection to their longer ghost lineages than previous scenarios have implied.