埃迪卡拉纪瓮安生物群中动物化石同步辐射三维无损成像研究

Since 1998, the phosphatized animal embryo fossils with cellular and sub-cellular structures preserved in three-dimensional detail from the Ediacaran Doushantuo Formation at Weng'an phosphate mining area (Guizhou, southwest China), have been considered as one of the oldest fossil metazoan records on the planet. During the last decade, studies of animal fossils from this unique Ediacaran taphonomic window have profoundly improved our outstanding of the evolution of metazoans in the Precambrian. However, these exquisite Doushantuo animal fossils remain to some extent controversial because of alleged taxonomic and ontogenetic biases. The majority of these Doushantuo animal fossils initially discovered were early cleavage-stage embryos undergoing equal holoblastic cell division which led to interpretation of giant sulfur bacteria and encysting protists, though many recent publications provide direct evidence of later stage Doushantuo embryos of a variety of forms, including some with bilaterally symmetric blastomere arrangements, micromere caps overlying macromere quartets or duets, clearly diverse cell types, and some cleavage stage forms with very particular structures still to be found in modern forms such as polar lobe-bearing embryos. Therefore, we propose this multidisciplinary study to elucidate the nature of these embryo-like microfossils from the Weng'an biota with the help of phase contrast-based synchrotron X-ray microtomography, and then try to figure out their biodiversity on the base of detailed investigation of diagenesis, morphology (including size, ornament patterns, capsule structures, cleavage patterns) and anatomy (including subcellular structures) of them. The expected data from this study will be useful to settle the debate,and more important, to evaluate the evolutionary level of multicellular animals during Doushantuo period from the Evo-Deve perspectives.

埃迪卡拉纪是多细胞动物起源和早期演化的关键时期，我国贵州瓮安生物群就是处于该时期的一个特异埋藏化石库，它以大量产出三维立体保存的动物胚胎化石而著称，是全球迄今最古老的后生动物化石记录之一，不仅为研究后生动物起源和早期演化过程提供了独一无二的实证材料，同时还为追溯"寒武纪大爆发"的源头打开了一扇新的窗口。然而瓮安生物群中究竟有多少种类的胚胎化石？它们又代表了哪些动物类群？两侧对称动物在瓮安生物群中是否出现？目前这些关键的科学问题仍有待回答或者存在争议。本项目将以瓮安生物群中的微体动物化石为研究对象，采用同步辐射X射线相衬显微断层成像技术，并结合传统的扫描电镜和岩石切片观察，开展瓮安动物化石埋藏学、形态学和比较解剖学研究，以期揭示这些动物化石的亲缘关系和多样性。这对探讨晚前寒武纪后生动物的演化水平具有重要意义，并将为审视"寒武纪大爆发"的本质提供新的视角。

动物的起源和早期演化一直是演化生物学和古生物学领域关注的前沿课题。距今约6.09亿年的埃迪卡拉纪瓮安生物群是迄今为止全球最古老的含有动物化石的特异埋藏化石库，为探索动物的早期演化历史提供了独一无二的实证材料。本项目借助同步辐射显微断层成像技术对来自埃迪卡拉纪瓮安生物群中可能和后生动物存在亲缘关系的化石进行三维无损复原研究，寻找早期后生动物胚胎期和成年个体期的化石证据，并通过比较生物学的视角识别出动物胚胎化石的多样性和亲缘关系。项目执行期间共使用同步辐射技术扫描了数以百计的微体化石标本，识别出海绵动物成体化石1种，各类动物胚胎化石3种，发表与主题密切相关的第一作者论文4篇，包括一区SCI论文3篇，含PNAS论文和Geology论文各1篇（均属于Nature Index论文）。项目得出的主要成果有：1.发现了迄今最古老的保存有细胞结构的海绵动物实体化石，这一保存精美的化石被命名为“贵州始杯海绵”，该发现一方面将海绵动物实体化石的记录向前推进大约七千万年，另一方面表明瓮安生物群种存在确凿无疑的动物，是迄今最古老的后生动物化石库，在研究动物起源和早期演化领域具有重要科学意义；2.首次在瓮安生物群中发现了不完全卵裂的动物胚胎化石，同步辐射复原结构表明其分裂方式和现代两侧对称动物胚胎中常见的盘状卵裂一致，表明两侧对称动物采用的胚胎发育机制很可能早在6亿年前就已经起源，同时也说明瓮安动物胚胎化石具有多样性；3.发现了保存有细胞分裂过程的胚胎化石标本，并据此对瓮安动物胚胎化石的埋藏过程进行了探讨，为了解胚胎化石的埋藏机制提出了新的思路；4.以动物胚胎化石为研究对象，将同步辐射断层成像技术和显微CT技术进行了对比研究，给出了两种技术各自的优缺点，为科研人员根据自身需求选择合适的研究手段提供了实证。