A reappraisal of the morphology and systematics of <i>Extuaspis megista</i>, a brachythoracid arthrodire from the Early Devonian of Yunnan, China

<i>Exutaspis megista</i> Y.-H. Liu and Wang 1981, discovered in the Lower Devonian Jiucheng Formation of Yunnan Province, south-western China, has been under phylogenetic contention since it was first described. Initially placed within Phlyctaeniidae, it was later suggested to be a member of Brachythoraci. Here, we re-examine the morphology of <i>Exutaspis</i> in detail, including the visceral surface of the skull roof and the dorsal aspect of the neurocranium, using the holotype and an additional specimen. Our redescription shows that <i>Exutaspis</i> possesses characters commonly seen in brachythoracids, such as an unbifid posterior postorbital process, the presence of a lateral consolidated arch and a developed nuchal thickening. Further, its morphology aligns closely with that of buchanosteoids, a cosmopolitan group of brachythoracids, particularly in the outline of the preorbital plates and in the dorsal profile of the neurocranium. To further elucidate the phylogenetic position of <i>Exutaspis</i>, we composed an updated matrix that contains 174 characters and 86 taxa, incorporating brachythoracids and comprehensively sampling arthrodires. The maximum parsimony analysis and Bayesian inference both recovered <i>Exutaspis</i> within the monophyletic Buchanosteoidea as the sister group of <i>Goodradigbeeon australianum</i>. The increasing knowledge of the diversity in the basal array of brachythoracids and their characters, such as the visceral consolidated structures on their skull roofs, provides important insight into the rise of brachythoracids from small bottom-feeders to fast-swimming giants.