Newly discovered fossils from the Silurian and Devonian periods are beginning to challenge embedded perceptions about the origin and early diversification of jawed vertebrates (gnathostomes). Nevertheless, an explicit cladistic framework for the relationships of these fossils relative to the principal crown lineages of the jawed vertebrates (osteichthyans: bony fishes and tetrapods; chondrichthyans: sharks, batoids, and chimaeras) remains elusive. We critically review the systematics and character distributions of early gnathostomes and provide a clearly stated hierarchy of synapomorphies covering the jaw-bearing stem gnathostomes and osteichthyan and chondrichthyan stem groups. We show that character lists, designed to support the monophyly of putative groups, tend to overstate their strength and lack cladistic corroboration. By contrast, synapomorphic hierarchies are more open to refutation and must explicitly confront conflicting evidence. Our proposed synapomorphy scheme is used to evaluate the status of the problematic fossil groups Acanthodii and Placodermi, and suggest profitable avenues for future research. We interpret placoderms as a paraphyletic array of stem-group gnathostomes, and suggest what we regard as two equally plausible placements of acanthodians: exclusively on the chondrichthyan stem, or distributed on both the chondrichthyan and osteichthyan stems.

中文翻译：

---

古生代有颌脊椎动物的特征

新发现的志留纪和泥盆纪化石开始挑战人们对有颌脊椎动物（颌口动物）起源和早期多样化的固有认知。然而，这些化石与有颌脊椎动物（骨鱼类：硬骨鱼和四足动物；软骨鱼类：鲨鱼、蝠鲼和银鲛）主要冠谱系之间关系的明确分支框架仍然难以捉摸。我们批判性地回顾了早期颌口动物的系统学和特征分布，并提供了一个清晰的共性层次，涵盖有颌茎颌口动物以及骨鱼和软骨鱼类干群。我们表明，旨在支持假定群体的单一性的字符列表往往会夸大其强度并且缺乏分支证实。相比之下，同源层次结构更容易被反驳，并且必须明确面对相互矛盾的证据。我们提出的共源性方案用于评估有问题的化石群棘皮鱼和盾皮鱼的状态，并为未来的研究提出有利可图的途径。我们将盾皮鱼解释为茎群颌口动物的并系排列，并提出我们认为棘皮动物的两种同样合理的位置：仅分布在软骨鱼茎上，或分布在软骨鱼和骨鱼茎上。