Stamford Museum and Nature Center, Stamford, CT, USA.
Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT, USA.
BMC Ecol Evol. 2022 Mar 21;22(1):34. doi: 10.1186/s12862-022-01985-z.
The consequences of the K-Pg mass extinction are reflected across present biodiversity, but many faunas that appeared immediately after the extinction event were very different from current ones. Choristodera is a clade of reptiles of uncertain phylogenetic placement that have an extremely poor fossil record throughout their 150-million-year history. Yet, choristoderes survived the K-Pg event and persisted until the Miocene.
I describe the skulls and skeletons of two new choristoderes from a single Paleocene ecosystem in western North America that reveal the hidden Cenozoic diversity of this reptile clade. Despite their similar size, the new species deviate dramatically in morphology. Kosmodraco magnicornis gen. et sp. nov. possesses an extremely short snout and extensive cranial ornamentation. The sacrum of K. magnicornis bears enlarged muscle attachment sites and other modifications reminiscent of some giant crocodylians. In contrast, Champsosaurus norelli sp. nov. is a longirostrine species with an uninflated and ventrally divergent postorbital skull. Together with a North American choristodere previously classified in the European genus Simoedosaurus, K. magnicornis substantiates a new clade of giant, short-snouted taxa endemic to the Americas. C. norelli is found to be an early-diverging member of the genus Champsosaurus from the Cretaceous-Paleogene of the northern hemisphere. This suggests the presence of several ghost lineages of champsosaurid that crossed the K-Pg boundary.
The new taxa greatly increase Cenozoic choristodere richness and strengthen the evidence for the existence of distinctive freshwater faunas in Paleogene Eurasia and North America, where this clade diversified to exploit newly available macropredatory niches in the aftermath of the asteroid impact. The new choristoderes also reveal the distinct ecological context in which extant freshwater predators of the Americas like alligatoroids and gars have their origins: Paleocene fluviolacustrine ecosystems in North America displayed high large predator diversity and morphological disparity relative to modern ones.
白垩纪-古近纪大灭绝的后果反映在现今的生物多样性中，但许多在灭绝事件后立即出现的动物群与现今的动物群有很大的不同。Choristodera 是一个不确定系统发育位置的爬行动物类群，在其 1.5 亿年的历史中化石记录极为稀少。然而， Choristodera 幸存了下来，并一直延续到中新世。
我描述了来自北美西部单一古新世生态系统的两个新 Choristodera 的头骨和骨骼，揭示了这个爬行动物类群在新生代的隐藏多样性。尽管它们的体型相似，但新物种在形态上却有很大的差异。Kosmodraco magnicornis gen. et sp. nov. 拥有一个极其短的吻部和广泛的头部装饰。K. magnicornis 的骶骨具有增大的肌肉附着点和其他类似于一些巨型鳄类的修改。相比之下，Champsosaurus norelli sp. nov. 是一种长吻类物种，具有未膨胀的和腹侧发散的眶后颅骨。与以前被归类于欧洲属 Simoedosaurus 的北美 Choristodera 一起，K. magnicornis 证实了一个新的巨型短吻类分类群，该分类群特有的美洲。C. norelli 被发现是北半球白垩纪-古近纪属 Champsosaurus 的早期分化成员。这表明存在几个穿越 K-Pg 边界的 Champsosaurid 幽灵谱系。
新分类群极大地增加了新生代 Choristodera 的丰富度，并加强了在古近纪欧亚大陆和北美的存在独特的淡水动物群的证据，在那里这个类群多样化，以利用小行星撞击后新出现的大型捕食性生态位。新的 Choristodera 还揭示了现存美洲淡水捕食者如短吻鳄和雀鳝起源的独特生态背景：北美的古新世河流湖泊生态系统与现代相比，大型捕食者的多样性和形态差异都很高。
