Department of Earth Sciences, The Natural History Museum, London SW7 5BD, United Kingdom;
Department of Earth Sciences, The Natural History Museum, London SW7 5BD, United Kingdom.
Proc Natl Acad Sci U S A. 2020 Apr 21;117(16):8966-8972. doi: 10.1073/pnas.1920733117. Epub 2020 Apr 6.
Identifying marine or freshwater fossils that belong to the stem groups of the major terrestrial arthropod radiations is a longstanding challenge. Molecular dating and fossils of their pancrustacean sister group predict that myriapods originated in the Cambrian, much earlier than their oldest known fossils, but uncertainty about stem group Myriapoda confounds efforts to resolve the timing of the group's terrestrialization. Among a small set of candidates for membership in the stem group of Myriapoda, the Cambrian to Triassic euthycarcinoids have repeatedly been singled out. The only known Devonian euthycarcinoid, from the Rhynie and Windyfield cherts hot spring complex in Scotland, reveals details of head structures that constrain the evolutionary position of euthycarcinoids. The head capsule houses an anterior cuticular tentorium, a feature uniquely shared by myriapods and hexapods. Confocal microscopy recovers myriapod-like characters of the preoral chamber, such as a prominent hypopharynx supported by tentorial bars and superlinguae between the mandibles and hypopharynx, reinforcing an alliance between euthycarcinoids and myriapods recovered in recent phylogenetic analysis. The Cambrian occurrence of the earliest euthycarcinoids supplies the oldest compelling evidence for an aquatic stem group for either Myriapoda or Hexapoda, previously a lacuna in the body fossil record of these otherwise terrestrial lineages until the Silurian and Devonian, respectively. The trace fossil record of euthycarcinoids in the Cambrian and Ordovician reveals amphibious locomotion in tidal environments and fills a gap between molecular estimates for myriapod origins in the Cambrian and a post-Ordovician crown group fossil record.
鉴定属于主要陆地节肢动物辐射的原始群体的海洋或淡水化石是一个长期存在的挑战。分子定年和它们的泛甲壳类姐妹群化石预测，多足类起源于寒武纪，比它们已知的最古老化石要早得多，但对原始多足类动物的不确定性阻碍了确定该群体陆地化时间的努力。在一小部分被认为是多足动物原始群体成员的候选者中，寒武纪到三叠纪的 euthycarcinoids 一再被单独挑出。唯一已知的泥盆纪 euthycarcinoids，来自苏格兰的 Rhynie 和 Windyfield 燧石温泉复合体，揭示了头部结构的细节，这些细节限制了 euthycarcinoids 的进化位置。头壳内有一个前面的角质膜顶篷，这是多足类和六足类所独有的特征。共聚焦显微镜恢复了前口腔室的多足类特征，例如由顶篷棒支撑的突出的咽下部和在颌骨和咽下部之间的 superlinguae，这加强了在最近的系统发育分析中恢复的 euthycarcinoids 和多足类之间的联盟。最早的 euthycarcinoids 出现在寒武纪，为多足类或六足类的原始水生群体提供了最古老的令人信服的证据，而在这两个陆生谱系的身体化石记录中，直到分别为志留纪和泥盆纪之前，这一直是一个空白。寒武纪和奥陶纪 euthycarcinoids 的痕迹化石记录揭示了潮汐环境中的两栖运动，并填补了分子估计的多足类起源与后奥陶纪冠群化石记录之间的空白。
