Abstract Stem‐group gnathostomes reveal the sequence of character acquisition in the origin of modern jawed vertebrates. The petalichthyids are placoderm‐grade stem‐group gnathostomes known from both isolated skeletal material and rarer articulated specimens of one genus. They are of particular interest because of anatomical resemblances with osteostracans, the jawless sister group of jawed vertebrates. Because of this, they have become central to debates on the relationships of placoderms and the primitive cranial architecture of gnathostomes. However, among petalichthyids, only the braincase of Macropetalichthys has been studied in detail, and the diversity of neurocranial morphology in this group remains poorly documented. Using X‐ray computed microtomography, we investigated the endocranial morphology of Shearsbyaspis oepiki Young, a three‐dimensionally preserved petalichthyid from the Early Devonian of Taemas‐Wee Jasper, Australia. We generated virtual reconstructions of the external endocranial surfaces, orbital walls and cranial endocavity, including canals for major nerves and blood vessels. The neurocranium of Shearsbyaspis resembles that of Macropetalichthys, particularly in the morphology of the brain cavity, nerves and blood vessels. Many characters, including the morphology of the pituitary vein canal and the course of the trigeminal nerve, recall the morphology of osteostracans. Additionally, the presence of a parasphenoid in Shearsbyaspis (previously not known with confidence outside of arthrodires and osteichthyans) raises some questions about current proposals of placoderm paraphyly. Our detailed description of this specimen adds to the known morphological diversity of petalichthyids, and invites critical reappraisal of the phylogenetic relationships of placoderms.

中文翻译：

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X 射线计算机显微断层扫描揭示了瓣鱼盾皮鱼 Shearsbyaspis oepiki Young 的神经颅解剖结构

摘要 干群有颚类动物揭示了现代有颚脊椎动物起源中的特征获取顺序。花瓣鱼是盾皮鱼级的茎类有颚类动物，从孤立的骨骼材料和一个属的稀有关节标本中都知道。它们之所以特别有趣，是因为它们在解剖学上与骨软骨动物（无颚的有颚脊椎动物的无颚姐妹群）有相似之处。正因为如此，它们已成为关于盾皮动物关系和有颚类动物原始颅骨结构的辩论的中心。然而，在petalichthyids 中，仅对Macropetalichthys 的脑壳进行了详细研究，并且该组中神经颅形态的多样性仍然很少被记录。使用 X 射线计算机显微断层扫描，我们研究了 Shearsbyaspis oepiki Young 的颅内形态，来自澳大利亚 Taemas-Wee Jasper 早泥盆世的三维保存的花瓣鱼。我们生成了颅内外部表面、眶壁和颅内腔的虚拟重建，包括主要神经和血管的管道。Shearsbyaspis 的神经颅与 Macropetalichthys 的神经颅相似，特别是在脑腔、神经和血管的形态上。许多特征，包括垂体静脉管的形态和三叉神经的走行，都让人联想到骨质的形态。此外，在 Shearsbyaspis 中存在一个副蝶骨（以前在 arthrodires 和 osteichthyans 之外没有信心知道），这引发了一些关于目前关于 placoderm paraphyly 提议的问题。