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

Exutaspis megista 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 Exutaspis 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 Exutaspis 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 Exutaspis, 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 Exutaspis within the monophyletic Buchanosteoidea as the sister group of Goodradigbeeon australianum. 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.大外盾鱼（*Exutaspis megista* Y.-H. Liu et Wang, 1981）发现于中国西南部云南省下泥盆统九城组（Lower Devonian Jiucheng Formation），自其首次被描述以来，系统发育归属便一直存在争议。该类群最初被归入褶甲鱼科（Phlyctaeniidae），后续研究则提出其应隶属于短胸类（Brachythoraci）。本研究基于正模标本与另一额外标本，对大外盾鱼的形态特征进行了详细重新检视，包括颅顶骨的内脏面与神经颅的背侧结构。本次重新描述显示，大外盾鱼具备短胸类的典型特征，如不分叉的后眶突、外侧联合弓的存在以及发育良好的项部增厚区。此外，其形态特征与布坎斯顿类（buchanosteoids）——一类广布的短胸类类群——高度吻合，尤其是在眶前板轮廓与神经颅的背视轮廓方面。为进一步明确大外盾鱼的系统发育位置，本研究构建了包含174个形态特征与86个分类单元的更新矩阵，纳入了短胸类类群并全面覆盖节甲鱼目（Arthrodira）类群。最大简约法分析与贝叶斯推断均显示，大外盾鱼隶属于单系的布坎斯顿超科，且为澳大利亚古德兰吉比鱼（*Goodradigbeeon australianum*）的姊妹群。对短胸类基干类群多样性及其特征（如颅顶骨的内脏联合结构）的深入认知，为阐明短胸类从小型底栖食者演化为高速游泳巨型类群的演化历程提供了重要见解。