Data from: An exceptionally preserved Late Devonian actinopterygian provides a new model for primitive cranial anatomy in ray-finned fishes

Actinopterygians (ray-finned fishes) are the most diverse living osteichthyan (bony vertebrate) group, with a rich fossil record. However, details of their earliest history during the middle Palaeozoic (Devonian) ‘Age of Fishes' remains sketchy. This stems from an uneven understanding of anatomy in early actinopterygians, with a few well-known species dominating perceptions of primitive conditions. Here we present an exceptionally preserved ray-finned fish from the Late Devonian (Middle Frasnian, ca 373 Ma) of Pas-de-Calais, northern France. This new genus is represented by a single, three-dimensionally preserved skull. CT scanning reveals the presence of an almost complete braincase along with near-fully articulated mandibular, hyoid and gill arches. The neurocranium differs from the coeval Mimipiscis in displaying a short aortic canal with a distinct posterior notch, long grooves for the lateral dorsal aortae, large vestibular fontanelles and a broad postorbital process. Identification of similar but previously unrecognized features in other Devonian actinopterygians suggests that aspects of braincase anatomy in Mimipiscis are apomorphic, questioning its ubiquity as stand-in for generalized actinopterygian conditions. However, the gill skeleton of the new form broadly corresponds to that of Mimipiscis, and adds to an emerging picture of primitive branchial architecture in crown gnathostomes. The new genus is recovered in a polytomy with Mimiidae and a subset of Devonian and stratigraphically younger actinopterygians, with no support found for a monophyletic grouping of Moythomasia with Mimiidae.

辐鳍鱼（Actinopterygians，ray-finned fishes）是现存多样性最高的硬骨脊椎动物（osteichthyan，bony vertebrate）类群，拥有极为丰富的化石记录。然而其在古生代中期泥盆纪“鱼类时代”的早期演化历史细节仍较为模糊。这一现状源于学界对早期辐鳍鱼类解剖结构的认知不均衡，少数广为人知的物种主导了学界对其原始形态的普遍认知。本研究报道一件发现于法国北部加来海峡省晚泥盆世（中弗拉斯期，距今约3.73亿年）的保存极为完好的辐鳍鱼化石。该新属仅由一件三维保存的头骨化石代表。CT扫描结果显示，该化石拥有近乎完整的脑颅（braincase），以及几乎完全关节连接的下颌弓（mandibular）、舌弓（hyoid）与鳃弓（gill arches）。该新属的脑颅（neurocranium）与同时期的米米鱼（Mimipiscis）存在显著差异：其具有短的主动脉管（aortic canal）及清晰的后切迹、为背侧主动脉预留的长沟槽、大型前庭囟（vestibular fontanelles）以及宽阔的眶后突（postorbital process）。在其他泥盆纪辐鳍鱼类中识别出类似但此前未被报道的解剖特征，表明米米鱼脑颅的部分解剖结构属于衍征（apomorphic），这对其作为辐鳍鱼类原始形态通用替代类群的普遍性提出了质疑。不过，该新属的鳃骨骼结构与米米鱼大体一致，这为冠群有颌脊椎动物（crown gnathostomes）的原始鳃弓架构研究增添了新的实证依据。系统发育分析显示，该新属与米米鱼科（Mimiidae）、部分泥盆纪及地层年代更晚的辐鳍鱼类构成多歧支（polytomy），且未发现支持莫氏鱼（Moythomasia）与米米鱼科构成单系群的证据。