Redescription and phylogenetic assessment of ‘<i>Prognathodon</i>’ <i>stadtmani</i>: implications for Globidensini monophyly and character homology in Mosasaurinae

<i>Prognathodon stadtmani</i> was discovered in the Campanian portion of the Upper Cretaceous Mancos Shale in western Colorado in 1975. Following its original description almost two decades ago, more material of the holotype was prepared. Those new elements include a quadrate, often considered one of the most critical bones for understanding mosasaur systematics, portions of the skull roof, posterior mandible, and braincase. <i>Prognathodon stadtmani</i> exhibits a suite of features that include both derived character states observed in later mosasaurines and primitive features present in earlier members of the clade, such as <i>Clidastes.</i> Although the quadrate resembles those of <i>Prognathodon</i> and <i>Globidens</i> in some respects, it lacks fusion between the suprastapedial process and ascending tympanic rim observed in those taxa. I ran a series of phylogenetic analyses using parsimony and Bayesian inference to understand the relationships of this taxon. Regardless of character selection or methodology, ‘<i>Prognathodon</i>’ <i>stadtmani</i> was not found to be monophyletic with the rest of the genus <i>Prognathodon</i>. Depending on method, it was recovered as the sister taxon to the rest of Globidensini, the sister taxon to Mosasaurini + <i>Globidens</i>, or an early diverging mosasaurine outside of both Globidensini and Mosasaurini. Owing to these results, I assign the type to a new genus, <i>Gnathomortis</i>, gen. nov., yielding the species <i>Gnathomortis stadtmani</i>, comb. nov. Furthermore, the rest of the genus <i>Prognathodon</i> and the clade Globidensini more broadly were not consistently recovered as monophyletic. Many characters of the quadrate and the dentition that typically united those clades are optimized as homoplastic based on my resulting topologies.1975年，科研人员于科罗拉多州西部上白垩统曼科斯页岩（Mancos Shale）的坎潘阶（Campanian）地层中发现了施塔特曼前凸齿龙（*Prognathodon stadtmani*）。在其最初正式描述发表近二十年后，该物种的正模标本（holotype）的更多化石材料得到修复整理。新发现的化石元素包括一块方骨（quadrate）——该骨骼常被视为理解沧龙（mosasaur）分类系统学的关键骨骼之一——以及颅顶（skull roof）、下颌后部（posterior mandible）和脑颅（braincase）的部分遗骸。施塔特曼前凸齿龙兼具两类特征：一类是后期沧龙亚科（mosasaurines）所具有的衍征（derived character states），另一类则是该演化支（clade）早期成员（如克丽斯塔沧龙*Clidastes*）所保留的祖征（primitive features）。尽管其方骨在部分形态特征上与前凸齿龙属（*Prognathodon*）和球齿龙属（*Globidens*）的方骨相似，但该物种的方骨并不具备上述类群中所见的上镫骨突（suprastapedial process）与鼓膜上升缘（ascending tympanic rim）的愈合结构。为明确该物种的系统发育关系，笔者采用简约法（parsimony）与贝叶斯推断（Bayesian inference）开展了多组系统发育分析（phylogenetic analyses）。无论特征选择方案或分析方法如何，‘前凸齿龙属’施塔特曼前凸齿龙（'*Prognathodon*' *stadtmani*）均未被归入前凸齿龙属（*Prognathodon*）的单系群（monophyletic group）中。根据不同分析方法，该物种被恢复为三种系统发育位置：球齿龙族（Globidensini）其余类群的姊妹群（sister taxon）、沧龙族（Mosasaurini）+球齿龙属（*Globidens*）的姊妹群，或是球齿龙族与沧龙族之外的早期分化沧龙亚科类群。基于上述结果，笔者将该物种的正模标本归入一新属——格诺莫提斯沧龙属（*Gnathomortis*），gen. nov.，由此形成新组合物种施塔特曼格诺莫提斯沧龙（*Gnathomortis stadtmani*），comb. nov.。此外，前凸齿龙属（*Prognathodon*）整体以及广义的球齿龙族（Globidensini）均未被稳定恢复为单系群。基于本次分析得到的拓扑结构，以往用于联合上述类群的诸多方骨与齿列特征均被优化为趋同演化特征（homoplastic character）。