The Giant Cretaceous Coelacanth (Actinistia, Sarcopterygii) Megalocoelacanthus dobiei Schwimmer, Stewart & Williams, 1994, and Its Bearing on Latimerioidei Interrelationships

We present a redescription of Megalocoelacanthus dobiei, a giant fossil coelacanth from Upper Cretaceous strata of North America. Megalocoelacanthus has been previously described on the basis of composite material that consisted of isolated elements. Consequently, many aspects of its anatomy have remained unknown as well as its phylogenetic relationships with other coelacanths. Previous studies have suggested that Megalocoelacanthus is closer to Latimeria and Macropoma than to Mawsonia. However, this assumption was based only on the overall similarity of few anatomical features, rather than on a phylogenetic character analysis. A new, and outstandingly preserved specimen from the Niobrara Formation in Kansas allows the detailed description of the skull of Megalocoelacanthus and elucidation of its phylogenetic relationships with other coelacanths. Although strongly flattened, the skull and jaws are well preserved and show many derived features that are shared with Latimeriidae such as Latimeria, Macropoma and Libys. Notably, the parietonasal shield is narrow and flanked by very large, continuous vacuities forming the supraorbital sensory line canal. Such an unusual morphology is also known in Libys. Some other features of Megalocoelacanthus, such as its large size and the absence of teeth are shared with the mawsoniid genera Mawsonia and Axelrodichthys. Our cladistic analysis supports the sister-group relationship of Megalocoelacanthus and Libys within Latimeriidae. This topology suggests that toothless, large-sized coelacanths evolved independently in both Latimeriidae and Mawsoniidae during the Mesozoic. Based on previous topologies and on ours, we then review the high-level taxonomy of Latimerioidei and propose new systematic phylogenetic definitions.本研究对北美上白垩统地层产出的巨型化石腔棘鱼（coelacanth）多氏巨腔棘鱼（Megalocoelacanthus dobiei）进行重新描述。此前，巨腔棘鱼属的研究仅基于由零散化石标本组成的复合标本展开，因此其解剖结构的诸多特征以及与其他腔棘鱼的系统发育关系至今仍不明确。既往研究认为，巨腔棘鱼属与矛尾鱼属（Latimeria）、大盖鱼属（Macropoma）的亲缘关系近于与莫森鱼属（Mawsonia）的亲缘关系，但该推论仅基于少量解剖特征的整体相似性，而非系统发育性状分析。此次发现于堪萨斯州尼奥布拉拉组（Niobrara Formation）的保存极为完好的新标本，使得我们能够详细描述巨腔棘鱼的头骨，并阐明其与其他腔棘鱼的系统发育关系。尽管头骨与颌骨严重受压扁平，但仍保存完好，并展现出诸多与矛尾鱼科（Latimeriidae，包含矛尾鱼属、大盖鱼属以及利比斯鱼属（Libys））共有的衍征。值得注意的是，其顶鼻盾（parietonasal shield）狭窄，两侧伴随有构成眶上感觉管的巨大连续腔隙，这种特殊形态在利比斯鱼属中也存在。巨腔棘鱼属的其他部分特征，如体型巨大、无齿，也与莫森鱼科（Mawsoniidae）的莫森鱼属、阿克塞尔罗德鱼属（Axelrodichthys）相一致。我们的分支系统学分析结果支持巨腔棘鱼属与利比斯鱼属在矛尾鱼科内构成姐妹群关系。该拓扑结构表明，无齿大体型的腔棘鱼在中生代的矛尾鱼科与莫森鱼科中独立演化而来。基于既往研究与本次研究得到的拓扑结构，我们对矛尾鱼亚目（Latimerioidei）的高级分类学进行了系统梳理，并提出了全新的系统发育定义。