State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China.
Center for Excellence in Life and Palaeoenvironment, Chinese Academy of Sciences, Nanjing, China.
Nature. 2020 Dec;588(7836):101-105. doi: 10.1038/s41586-020-2883-7. Epub 2020 Nov 4.
Resolving the early evolution of euarthropods is one of the most challenging problems in metazoan evolution. Exceptionally preserved fossils from the Cambrian period have contributed important palaeontological data to deciphering this evolutionary process. Phylogenetic studies have resolved Radiodonta (also known as anomalocaridids) as the closest group to all euarthropods that have frontalmost appendages on the second head segment (Deuteropoda). However, the interrelationships among major Cambrian euarthropod groups remain disputed, which impedes our understanding of the evolutionary gap between Radiodonta and Deuteropoda. Here we describe Kylinxia zhangi gen. et. sp. nov., a euarthropod from the early Cambrian Chengjiang biota of China. Kylinxia possesses not only deuteropod characteristics such as a fused head shield, a fully arthrodized trunk and jointed endopodites, but also five eyes (as in Opabinia) as well as radiodont-like raptorial frontalmost appendages. Our phylogenetic reconstruction recovers Kylinxia as a transitional taxon that bridges Radiodonta and Deuteropoda. The most basal deuteropods are retrieved as a paraphyletic lineage that features plesiomorphic raptorial frontalmost appendages and includes Kylinxia, megacheirans, panchelicerates, 'great-appendage' bivalved euarthropods and isoxyids. This phylogenetic topology supports the idea that the radiodont and megacheiran frontalmost appendages are homologous, that the chelicerae of Chelicerata originated from megacheiran great appendages and that the sensorial antennae in Mandibulata derived from ancestral raptorial forms. Kylinxia thus provides important insights into the phylogenetic relationships among early euarthropods, the evolutionary transformations and disparity of frontalmost appendages, and the origin of crucial evolutionary innovations in this clade.
解析后生动物的早期演化是后生动物演化中最具挑战性的问题之一。来自寒武纪的保存异常完好的化石为破译这一演化过程提供了重要的古生物学数据。系统发育研究将放射虫动物门（也称为奇虾类）确定为与所有具有第二个头部节段前触角的真节肢动物（后真节肢动物）最接近的类群。然而，主要的寒武纪真节肢动物群体之间的相互关系仍然存在争议，这阻碍了我们对放射虫动物门和后真节肢动物之间的进化差距的理解。本文描述了一种来自中国早寒武世澄江生物群的真节肢动物——张氏克拉辛虾。张氏克拉辛虾不仅具有后真节肢动物的特征，如头甲愈合、体躯完全关节化和具关节的内附肢，还具有五只眼睛（如欧巴宾海蝎）和放射虫状的捕食性前触角。我们的系统发育重建结果表明，张氏克拉辛虾是连接放射虫动物门和后真节肢动物的过渡类群。最基干的后真节肢动物被重建为一个并系群，具有原始的捕食性前触角，包括张氏克拉辛虾、巨型栉蚕、泛甲壳类、“大附肢”双瓣真节肢动物和栉蚕目。这种系统发育拓扑结构支持以下观点：放射虫动物门和巨型栉蚕的前触角是同源的，螯肢动物门的螯肢起源于巨型栉蚕的大附肢，而真节肢动物门头附器是从祖先的捕食形式演化而来的。张氏克拉辛虾因此为早期真节肢动物的系统发育关系、前触角的演化转变和多样性，以及这个类群中关键进化创新的起源提供了重要的见解。
