Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL 60637-1508, USA
UCD School of Biology and Environmental Science, UCD Science Education and Research Centre (West), UCD Earth Institute, University College Dublin, Belfield, Dublin 4, Ireland.
Proc Biol Sci. 2018 Jan 10;285(1870). doi: 10.1098/rspb.2017.2418.
Although relationships among the major groups of living gnathostomes are well established, the relatedness of early jawed vertebrates to modern clades is intensely debated. Here, we provide a new description of , a Middle Devonian (Givetian approx. 385-million-year-old) stem chondrichthyan from Germany, and one of the very few early chondrichthyans in which substantial portions of the endoskeleton are preserved. Tomographic and histological techniques reveal new details of the gill skeleton, hyoid arch and jaws, neurocranium, cartilage, scales and teeth. Despite many features resembling placoderm or osteichthyan conditions, phylogenetic analysis confirms as a stem chondrichthyan and corroborates hypotheses that all acanthodians are stem chondrichthyans. The unfamiliar character combination displayed by , alongside conditions observed in acanthodians, implies that pre-Devonian stem chondrichthyans are severely under-sampled and strongly supports indications from isolated scales that the gnathostome crown group originated at the latest by the early Silurian (approx. 440 Ma). Moreover, phylogenetic results highlight the likely convergent evolution of conventional chondrichthyan conditions among earliest members of this primary gnathostome division, while skeletal morphology points towards the likely suspension feeding habits of , suggesting a functional origin of the gill slit condition characteristic of the vast majority of living and fossil chondrichthyans.
虽然主要的有颌类动物群体之间的关系已经得到很好的确立，但早期有颌脊椎动物与现代进化枝的亲缘关系仍存在激烈的争论。在这里，我们提供了来自德国的中泥盆世（吉维特阶，约 3.85 亿年前）软骨鱼类化石的新描述，这是为数不多的保存了大量内骨骼的早期软骨鱼类之一。断层扫描和组织学技术揭示了鳃骨架、舌弓和颌骨、颅神经、软骨、鳞片和牙齿的新细节。尽管有许多特征类似于盾皮鱼类或硬骨鱼类的条件，但系统发育分析证实 是一种软骨鱼类的原始类型，并支持了所有棘鱼类都是软骨鱼类原始类型的假说。尽管 表现出许多不熟悉的特征组合，与棘鱼类观察到的条件相似，但这表明前泥盆纪的软骨鱼类原始类型严重采样不足，并强烈支持从孤立的鳞片中得出的结论，即有颌类冠群的起源时间最晚在早志留世（约 4.40 亿年前）。此外，系统发育结果突出了最早的有颌类原始分支成员中常规软骨鱼类条件的可能趋同进化，而骨骼形态则指向 的可能是悬浮摄食习惯，这表明绝大多数现存和化石软骨鱼类的鳃裂条件具有功能起源。
