The neurocranium is an integral part of the vertebrate head, itself a major evolutionary innovation^1,2. However, its early history remains poorly understood, with great dissimilarity in form between the two living vertebrate groups: gnathostomes (jawed vertebrates) and cyclostomes (hagfishes and lampreys)^2,3. The 100 Myr gap separating the Cambrian appearance of vertebrates^4,5,6 from the earliest three-dimensionally preserved vertebrate neurocrania⁷ further obscures the origins of modern states. Here we use computed tomography to describe the cranial anatomy of an Ordovician stem-group gnathostome: Eriptychius americanus from the Harding Sandstone of Colorado, USA⁸. A fossilized head of Eriptychius preserves a symmetrical set of cartilages that we interpret as the preorbital neurocranium, enclosing the fronts of laterally placed orbits, terminally located mouth, olfactory bulbs and pineal organ. This suggests that, in the earliest gnathostomes, the neurocranium filled out the space between the dermal skeleton and brain, like in galeaspids, osteostracans and placoderms and unlike in cyclostomes². However, these cartilages are not fused into a single neurocranial unit, suggesting that this is a derived gnathostome trait. Eriptychius fills a major temporal and phylogenetic gap in our understanding of the evolution of the gnathostome head, revealing a neurocranium with an anatomy unlike that of any previously described vertebrate.

神经颅骨是脊椎动物头部的一个组成部分，其本身就是一项重大的进化创新^1,2。然而，人们对它的早期历史仍然知之甚少，现存的两个脊椎动物类群：颚口类（有颌类脊椎动物）和圆口类（盲鳗和七鳃鳗）在形态上存在很大差异^2,3。寒武纪脊椎动物的出现^4,5,6与最早的三维保存的脊椎动物神经颅⁷之间有 100 密尔的差距，这进一步模糊了现代国家的起源。在这里，我们使用计算机断层扫描来描述奥陶纪干群颌骨的颅骨解剖结构：来自美国科罗拉多州哈丁砂岩的Eriptychius americanus ⁸。埃里普提奇乌斯的头部化石保存了一组对称的软骨，我们将其解释为眶前神经颅，包围横向放置的眼眶的前部、末端的嘴、嗅球和松果体。这表明，在最早的颌口动物中，神经颅骨填充了真皮骨骼和大脑之间的空间，就像在鞘甲鱼、骨甲纲和盾皮动物中一样，而与圆口动物不同²。然而，这些软骨并未融合成单个神经颅单元，表明这是衍生的颌口动物特征。Eriptychius填补了我们对颌口头部进化的理解中的一个主要的时间和系统发育空白，揭示了其神经颅骨的解剖结构不同于任何先前描述的脊椎动物。