Our understanding of the earliest evolution of jawed vertebrates depends on a credible phylogenetic assessment of the jawed stem gnathostomes collectively known as ‘placoderms’. However, their relationships, and even whether ‘placoderms’ represent a single radiation or a paraphyletic array, remain contentious. Here we describe the endocranial cavity and inner ear of Brindabellaspis stensioi, commonly recovered as a taxon of uncertain affinity branching near the base of ‘placoderms’. While some features of its braincase and endocast resemble those of jawless vertebrates, its inner ear displays a repertoire of crown gnathostome characters. Both parsimony and Bayesian analyses suggest that established hypotheses of ‘placoderm’ relationships are unstable, with newly-revealed anatomy pointing to a potentially radical revision of early gnathostome evolution. Our results call into question the appropriateness of fusiform ‘placoderms’ as models of primitive gnathostome anatomy and raise questions of homology relating to key cranial features.

中文翻译：

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有颌脊椎动物的内模和骨迷宫揭示了最早的有颌脊椎动物的多样化

我们对下颚脊椎动物最早进化的理解取决于对统称为“板皮动物”的下颚茎有颚类的可信系统发育评估。然而，它们的关系，甚至“盾皮”是代表单一辐射还是并系阵列，仍然存在争议。在这里，我们描述了Brindabellaspis stensioi 的颅腔和内耳，通常作为一种在“placoderms”基部附近分枝的不确定亲和性分类群恢复。虽然它的脑壳和内模的某些特征类似于无颚脊椎动物的特征，但它的内耳显示出一系列冠有颚动物的特征。简约和贝叶斯分析都表明，“板皮”关系的既定假设是不稳定的，新发现的解剖结构表明对早期有颚类动物进化的潜在彻底修正。我们的研究结果对梭形“盾皮”作为原始有颚体解剖模型的适当性提出了质疑，并提出了与关键颅骨特征相关的同源性问题。