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 Diplocynodon and the crocodyloid Asiatosuchus but also a less well-understood group known as the Planocraniidae. 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. Dentaneosuchus gen. nov. is proposed to designate the taxon originally described as Atacisaurus crassiproratus. A phylogenetic analysis recovers Dentaneosuchus crassiproratus 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, Dentaneosuchus crassiproratus rivalled in size with Barinasuchus arveloi 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）界线处的陆地生态系统，孕育了一类与现代半水生支系亲缘关系密切的特化新鳄类（neosuchians）类群组合，涵盖短吻鳄超科的双脊鳄属（Diplocynodon）、鳄超科的亚洲鳄属（Asiatosuchus），以及一类认知程度较低的平颅鳄科（Planocraniidae）类群。值得注意的是，欧洲的动物群组成独具特色：这里还保存了诺托鳄类（Notosuchia）的部分最晚代表类群——该类群此前曾在白垩纪的陆地生态系统中经历过成功的辐射演化，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，但它们与K–Pg界线后的类群的亲缘关系尚未明确，部分原因是在地层界线两侧，该类群的化石均较为稀少且保存破碎。古近纪欧洲的西贝鳄类化石曾在法国、德国、葡萄牙与西班牙的始新世地层中被报道。厘清它们的系统发育位置，或可为揭示其疑似的冈瓦纳亲缘关系提供关键线索。基于新发现的颅骨及颅后遗骸，我们对迄今最完整、体型最大的欧洲西贝鳄类展开了描述，该标本发现于法国南部伊瑟尔（Issel）与雷阿尔蒙（Réalmont）的中始新世（巴顿期）地层中。本文建立新属丹特诺苏鳄属（Dentaneosuchus gen. nov.），以指代原被定为厚吻阿塔西斯鳄（Atacisaurus crassiproratus）的分类单元。系统发育分析显示，厚吻丹特诺苏鳄（Dentaneosuchus crassiproratus）为西贝鳄科（Sebecidae）最基部分支的类群；而西贝鳄科此前仅在南美洲古近纪及中新世地层中发现大量标本。在K–Pg界线后的西贝鳄类中，厚吻丹特诺苏鳄的体型可与委内瑞拉中新世的巴里纳苏鳄（Barinasuchus arveloi）相媲美，这表明西贝鳄科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。