Anatomy and phylogeny of an exceptionally large sebecid (Crocodylomorpha) from the middle Eocene of southern France

Post Cretaceous–Paleogene (K–Pg) boundary continental ecosystems of Europe host a distinctive assemblage of derived neosuchians related to modern semi-aquatic lineages, including the alligatoroid <i>Diplocynodon</i> and the crocodyloid <i>Asiatosuchus</i> but also a less well-understood group known as the Planocraniidae<i>.</i> Yet, uniquely in its faunal composition, Europe also preserves some of the last representatives of Notosuchia, a group that previously underwent a successful radiation in terrestrial ecosystems of the Cretaceous, especially in Gondwana. Although Europe harbored a few members of Sebecosuchia during the Cretaceous, their affinities with post-K–Pg forms is not ascertained, in part due to their rare and fragmentary nature on both sides of the stratigraphic boundary. Paleogene European sebecosuchians have been reported from the Eocene of France, Germany, Portugal, and Spain. Resolving their phylogenetic position may shed light on their suspected Gondwanan affinities. Based on new cranial and postcranial remains, we provide a description of the most complete and largest European sebecosuchian to date from the middle Eocene (Bartonian) of Issel and Réalmont, southern France. <i>Dentaneosuchus</i> gen. nov. is proposed to designate the taxon originally described as <i>Atacisaurus crassiproratus</i>. A phylogenetic analysis recovers <i>Dentaneosuchus crassiproratus</i> as the basalmost branching member of Sebecidae, a clade otherwise known from abundant specimens from the Paleogene and Miocene of South America. Among post-K–Pg sebecids, <i>Dentaneosuchus crassiproratus</i> rivalled in size with <i>Barinasuchus arveloi</i> from the Miocene of Venezuela, demonstrating that sebecids achieved the status of apex predators in terrestrial ecosystems of both Europe and South America.

欧洲白垩纪-古近纪（K–Pg）界线后的大陆生态系统中，孕育了一类与现代半水生鳄类支系亲缘关系紧密的特化新鳄类（neosuchians）类群，涵盖短吻鳄超科（alligatoroid）的*Diplocynodon*（双脊鳄属）、鳄超科（crocodyloid）的*Asiatosuchus*（亚洲鳄属），以及一类认知程度较低的平颅鳄科（Planocraniidae）类群。而在动物群组成上，欧洲独具特色：这里还保存了诺托鳄类（Notosuchia）的部分最晚残存类群。该类群曾在白垩纪的陆地生态系统中经历过一次成功的辐射演化，尤以冈瓦纳大陆（Gondwana）地区最为繁盛。尽管欧洲在白垩纪时期曾存在少量西贝鳄亚目（Sebecosuchia）成员，但该类群与K–Pg界线后的类群的亲缘关系尚未明确，部分原因是该类群在地层界线两侧的化石均极为稀少且破碎不全。古近纪的欧洲西贝鳄亚目类群化石，已在法国、德国、葡萄牙与西班牙的始新世（Eocene）地层中被发现并报道。明确该类群的系统发育位置，有助于阐明其被推测的冈瓦纳亲缘关系。基于新发现的颅骨与颅后骨骼（postcranial）化石标本，本文对产自法国南部Issel与Réalmont地区中始新世（巴顿期，Bartonian）的、目前已知最完整且体型最大的欧洲西贝鳄亚目类群展开了描述。本文建立新属*Dentaneosuchus*（gen. nov.），以涵盖最初被命名为*Atacisaurus crassiproratus*的分类群（taxon）。系统发育分析结果显示，*Dentaneosuchus crassiproratus*是西贝鳄科（Sebecidae）的基部分支类群；该演化支的其他类群则以南美洲古近纪及中新世（Miocene）地层中产出的大量标本为代表。在K–Pg界线后的西贝鳄科类群中，*Dentaneosuchus crassiproratus*的体型可与产自委内瑞拉中新世的*Barinasuchus arveloi*（巴里纳斯鳄）相媲美，这表明西贝鳄科类群曾在欧洲与南美洲的陆地生态系统中均占据顶级捕食者的生态位。