寒武纪早期四方塔型壳属的胚胎发育及其系统学意义

The embryonic development of the early Cambrian Quadrapyrgites and its phylogenetic implication

寒武纪大爆发是生命演化史上重要的创新事件。我国澄江动物群(距今约5.18亿年)代表了寒武纪大爆发的高潮,而产自寒武系最底部,时代上早于澄江动物群的宽川铺生物群(距今约5.35亿年)则为研究寒武纪大爆发的第一幕提供了重要的埋藏窗口。宽川铺生物群产自陕南宁强、西乡等地寒武系底部宽川铺组钙质磷块岩和磷质碎屑灰岩中,以保存了大量动物胚胎和软躯体微型动物而闻名。其中四方塔型壳(Quadrapyrgites)是宽川铺生物群中常见的软躯体动物化石,因与同层位的橄榄蛋(Olivooides)在发育过程和形态上具有相似性而被认为是后者的姐妹群。但它们在动物树上的位置仍然存在争议。为检验前人提出的各种亲缘关系假说,本文综合使用电子扫描显微镜成像和超高分辨率X射线显微断层成像技术,重建了四方塔型壳胚胎标本的三维结构,首次揭示了其内陷式原肠作用类型。新的数据支持了四方塔型壳属于刺细胞动物而非环神经动物的观点。

The Cambrian explosion is one of the most important biological innovations in life history.The Chengjiang Biota(518 Ma)from South China records the last major episode of the big bang of life,while the Kuanchuanpu Biota(535 Ma)from the lower-most Cambrian,which is earlier than the Chengjiang Biota,provides a crucial taphonomic window for investigating the first major episode of the Cambrian explosion.The Kuanchuanpu Biota discovered from the early Cambrian phosphatic limestone in Ningqiang and Xixiang Couties,Shaanxi Province,is well-known as it preserved abundant soft-bodied microscopic animals and associated embryos.Among these animal fossils,Quadrapyrgites is a very common taxon.It is widely thought to be a sister group of coeval Olivooides,because they share similar developmental process and body plan.However,the phylogenetic positions for both taxa remain uncertain.To test all the previous affinity hypothesis for Quadrapyrgites,herein we reconstructed the three-dimensional structures of several embryonic specimens of Quadrapyrgites,using scanning electronic microscopy and high-resolution X-ray tomographic microscopy.For the first time,we revealed that the gastrulation process of the embryos of Quadrapyrgites is typical invagination.The new data presented here suggests that the Quadrapyrgites and their sister groups should be more likely diploblastic cnidarians rather than triploblastic cycloneuralians.