First occurrence of a mawsoniid coelacanth in the Early Jurassic of Europe

ABSTRACT—Coelacanths form a clade of sarcopterygian fishes (lobe-finned vertebrates) that today is represented by a single genus, <i>Latimeria</i>. This genus belongs to a lineage of marine coelacanths, the latimeriids, whose fossils are common in the Jurassic and the Cretaceous deposits of Europe and North America. During the same periods, another lineage of fresh/brackish water coelacanths, the mawsoniids, occurred in South America, Africa, and Madagascar. Mawsoniids are supposed to have originated during the Triassic in North America and were assumed to have subsequently dispersed to South America during the Jurassic, before reaching western Africa during the Early Cretaceous. Previous hypotheses advocated that mawsoniid coelacanths reached Europe during the Late Cretaceous, suggesting the dispersal of freshwater organisms from Africa to Europe during this period. We here reevaluate this scenario based on the reexamination of the coelacanth <i>Trachymetopon</i> from the Early Jurassic of Germany. Although this genus is known from remarkably well-preserved material, its relationships to other Mesozoic coelacanths remained unsolved. An anatomical investigation shows that <i>Trachymetopon</i> shares many common features with the Late Jurassic–Late Cretaceous mawsoniids <i>Mawsonia</i> and <i>Axelrodichthys</i> from western Gondwana, such as the absence of a descending process of the supratemporal, the presence of ossified ribs, and the skull roof and cheek bones ornamented by conspicuous coarse rugosities and ridges. A phylogenetic analysis of morphological characters places <i>Trachymetopon</i> within Mawsoniidae. We suggest that mawsoniid coelacanths were already present in Europe from the Early Jurassic onwards, challenging previous paleobiogeographic scenarios.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

摘要——腔棘鱼（coelacanths）属于肉鳍鱼类（sarcopterygian fishes，即叶鳍脊椎动物（lobe-finned vertebrates））的一个演化支，现生类群仅存矛尾鱼属（Latimeria）。该属隶属于海洋腔棘鱼支系矛尾鱼科（latimeriids），其化石在欧洲与北美的侏罗纪（Jurassic）、白垩纪（Cretaceous）地层中广为分布。同期另一支淡水/半咸水腔棘鱼支系莫森鱼科（mawsoniids）则分布于南美洲、非洲及马达加斯加。
学界此前推测莫森鱼科起源于三叠纪（Triassic）的北美，并于侏罗纪时期扩散至南美，随后在早白垩世（Early Cretaceous）抵达西非。过往假说提出，晚白垩世（Late Cretaceous）时期莫森鱼科腔棘鱼曾抵达欧洲，暗示该时段存在淡水生物从非洲向欧洲的扩散事件。
本文基于对德国早侏罗世产出的腔棘鱼粗顶鱼属（Trachymetopon）的重新检视，重新评估了这一古生物地理假说场景。尽管该属拥有保存极为完好的化石材料，但其与其他中生代（Mesozoic）腔棘鱼的演化关系始终未获明确。
解剖学研究显示，粗顶鱼属与冈瓦纳（Gondwana）西部晚侏罗世至晚白垩世的莫森鱼科成员莫森鱼属（Mawsonia）、阿克塞尔罗德鱼属（Axelrodichthys）共享诸多共有特征：颞上骨无下降突、具骨化肋骨，颅骨顶区与颊部骨骼带有显著粗糙纹理及脊状突起。
基于形态学特征的系统发育分析将粗顶鱼属归入莫森鱼科。本文提出，莫森鱼科腔棘鱼早在早侏罗世就已分布于欧洲，这一结论对既往古生物地理假说场景提出了挑战。
补充数据——本文的补充材料可通过www.tandfonline.com/UJVP免费获取。