陕南、川北早寒武世胚胎化石的研究

The early Cambrian embryo fossils have a research history of just more than one decade, and are now one of the hot topics in evolutionary biology. They provide direct fossil evidence for the research on the morphology, development mode and evolution of the early metazoans. In recent years, they aroused fierce debate among palaeontologists and evolutionary biologists, and many different opinions about their affinities and development modes were proposed. This project is designed based on the previously accumulated, beautifully preserved embryo fossils of Olivooides, Quadrapyrgites and Pseudooides, and plans to recover as many delicately preserved embryo fossils and their hatchlings as possible from southern Shaanxi and northern Sichuan. In order to further probe the affinities and evolutionary significance of these embryo fossils, this project plans to establish more complete ontogenetic sequences of Olivooides, Quadrapyrgites and Pseudooides. Meanwhile, we plan to probe the possibly preserved internal biological structures of these embro fossils through Synchrotron Radiation X-ray Tomographic Microscopy (or other methods such as thin sectioning and Micro-CT analysis). And finally, we will discuss the most likely affinities of Olivooides, Quadrapyrgites and Pseudooides based on comprehensive data including morphology, ontogenetic mode, growth pattern and internal biological structures, as well as their bearing upon the evolution of the early metazoans.

早寒武世的胚胎化石有10多年的研究历史，是当前演化生物学研究的热点之一，它们为早期后生动物的形态、发育模式和演化方式的研究提供了直接的化石依据。近几年来，它们引起了古生物学家和演化生物学家的热烈讨论，围绕它们的亲缘关系展开了争论。为了深入探索这些胚胎化石的亲缘关系和演化意义，本项目拟在已经积累的大量精美保存的胚胎化石Olivooides、Quadrapyrgites和Pseudooides的基础之上，争取能够发现更多精美保存的胚胎化石和它们的胚后阶段的标本，建立Olivooides、Quadrapyrgites和Pseudooides的更完整的个体发育序列。利用同步辐射X射线层析显微技术，并辅以切片技术或Micro-CT技术，探索标本内部可能保存的生物结构。最后，综合形态学、个体发育模式、生长方式和内部生物结构的数据，探讨这些胚胎化石最有可能的亲缘关系和它们对早期生命演化的意义。

化石胚胎在陕南、川北寒武系幸运阶含磷岩层中广泛出现，有较好的研究基础，但是它们的亲缘关系一直存在争论，这极大的限制了对它们演化意义的研究。本研究在陕南宁强县石钟沟剖面、西乡县张家沟剖面和川北南江县新立剖面幸运阶第一小壳化石带中发现了丰富多样的磷酸盐化立体保存的化石胚胎。经过四年的研究，一共发表了原创性研究论文13篇，均为一作或通讯，其中SCI期刊论文12篇，中文核心期刊论文1篇，得到了以下认识：（1）化石胚胎的辐射对称数有较高的多样性。发现了二辐射的Pseudooides prima和它的胚后阶段Hexaconularia sichuanensis，四辐射的Quadrapyrgites quadratacris，五辐射的Olivooides multisulcatus和Qinscyphus necopinus，六辐射的Embryo I，和八辐射的Embryo II。此外还发现了大量光滑无饰的球状化石，代表可能的化石胚胎。有些化石胚胎的辐射状结构在光学显微镜下很明显，在扫描电镜下不明显。（2）报道了Olivooides和Quadrapyrgites已知最大的胚后个体，推测它们的成体应该在3厘米以上，可能具有至少50个环；（3）辐射对称支持幸运阶化石胚胎应该解释为刺胞动物，而不是环神经动物。化石胚胎多样的辐射对称数表明现生刺胞动物中二辐射和四辐射对称是它们幸运阶近亲的演化孑遗。（4）报道了新型刺胞动物意外秦钵水母（Qinscyphus necopinus）的胚胎和胚后阶段。秦钵水母是直接发育的。直接发育是幸运阶刺胞动物广泛采用的个体发育模式，有可能是后生动物较原始的个体发育模式，而间接发育是较进化的个体发育模式。（5）首次报道了刺胞动物辐射对称数的种内变异现象，表明刺胞动物调控辐射对称数种内变异机制在寒武系幸运阶就已经出现了。