最古老的动吻动物化石——始祖动吻虫

　　

　动吻动物门与兜甲动物门、腮曳动物门、线虫动物门、线形动物门、有爪动物门、缓步动物门和节肢动物门共同构成动物界最庞大的分支，即蜕皮动物。动吻动物门是一类较小型底栖生物，终生在海洋中生活，大约有240个现生种，迄今为止，没有化石记录，它的起源和演化一直是个谜。

　　近日，由中国科学院南京地质古生物研究所副研究员张华侨、美国弗吉尼亚理工大学教授肖书海和长安大学教授刘云焕等组成的联合研究团队在川北和陕南早寒武世（5.35亿年前）地层中发现了三维磷酸盐化保存的新的动物化石。这些新化石是早期的动吻动物，代表着动吻动物门在整个地质历史中已知的唯一的化石记录。其中一种命名为珍稀始祖动吻虫（Eokinorhynchus rarus），它的名字表明其稀缺性和与现生动吻动物之间的关系。它约2毫米长，有20个体节，每个体节上布满一圈（约20–40个）长方形的小骨板，有五对较大的具刺骨板对称分布，腹部有一个单独的具刺大骨板，每个大骨板横跨2–3个体节，有较多空心的刺随机分布在体节上。

　　始祖动吻虫与现生的动吻动物形态相似，表明它们之间有较近的演化关系。例如，始祖动吻虫与现生的动吻动物的躯干上都有空心的刺，体节数目少而且宽，体节上覆盖骨板。它们之间也有较大的差别，例如现生的动吻动物只有11个体节，刺较小，而始祖动吻虫有20个体节，骨板和刺都更大。因此，该研究认为始祖动吻虫属于动吻动物门的祖先类型，是该门类的第一个化石种。同时也表明，动吻动物门在寒武纪早期就已经出现，为“寒武纪大爆发”新添了一个动物门类。

　　该研究成果于11月26日发表于Nature的子期刊Scientific Reports。

　　论文相关信息：Huaqiao Zhang, Shuhai Xiao, Yunhuan Liu, Xunlai Yuan, Bin Wan, A. D. Muscente, Tiequan Shao, Hao Gong & Guohua Cao, 2015. Armored kinorhynch-like scalidophoran animals from the early Cambrian. Scientific Reports. Doi: 10.1038/srep16521.

始祖动吻虫复原图

始祖动吻虫扫描电镜照片

原文摘要：

Armored kinorhynch-like scalidophoran animals from the early Cambrian

Morphology-based phylogenetic analyses support the monophyly of the Scalidophora (Kinorhyncha, Loricifera, Priapulida) and Nematoida (Nematoda, Nematomorpha), together constituting the monophyletic Cycloneuralia that is the sister group of the Panarthropoda. Kinorhynchs are unique among living cycloneuralians in having a segmented body with repeated cuticular plates, longitudinal muscles, dorsoventral muscles, and ganglia. Molecular clock estimates suggest that kinorhynchs may have diverged in the Ediacaran Period. Remarkably, no kinorhynch fossils have been discovered, in sharp contrast to priapulids and loriciferans that are represented by numerous Cambrian fossils. Here we describe several early Cambrian (~535 million years old) kinorhynch-like fossils, including the new species Eokinorhynchus rarus and two unnamed but related forms. E. rarus has characteristic scalidophoran features, including an introvert with pentaradially arranged hollow scalids. Its trunk bears at least 20 annuli each consisting of numerous small rectangular plates, and is armored with five pairs of large and bilaterally placed sclerites. Its trunk annuli are reminiscent of the epidermis segments of kinorhynchs. A phylogenetic analysis resolves E. rarus as a stem-group kinorhynch. Thus, the fossil record confirms that all three scalidophoran phyla diverged no later than the Cambrian Period.