A new scalidophoran animal from the Cambrian Fortunian Stage of South China and its implications for the origin and early evolution of Kinorhyncha

摘要

Kinorhyncha is a cycloneuralian phylum with about 240 extant species, and it was not until 2015 that the first and only fossil kinorhynch species Eokinorhynchus rarus was reported. Here, we describe a new scalidophoran animal, Zhongpingscolex qinensis gen. a sp. nov., from the Cambrian Fortunian Kuanchuanpu Formation at Zhangjiagou section, southern Shaanxi Province, South China. The holotype and only specimen possesses morphology comparable to that of E. rarus. They both possess heterogeneous trunk annuli, and compared with E. rarus, Z. qinensis gen. a sp. nov. lacks a neck. Phylogenetic analysis resolves Z. qinensis gen. a sp. nov. as the sister group of E. rarus, both constituting stem-lineage derivatives of Kinorhyncha. The evolutionary scenario from the last common ancestor of crown-group Scalidophora to the last common ancestor of total-group Kinorhyncha includes the acquisition of a much shortened body with a relatively low body length to width ratio, heterogeneous trunk annuli, and macroannuli with simple armatures. The spines, large spinose sclerites, and annular rings of rectangular plates are autapomorphic to E. rarus, and segments (zonites) with independent cuticular plates as in crown-group kinorhynchs were evolved later.