徐淮地区新元古代早期宏体化石研究

The rise of complex multicellular organisms is one of the major evolutionary transitions in the history of life. Complex multicellular organisms began to radiate in the Ediacaran Period, and dominated the biosphere during the Phanerozoic. Molecular clock estimates suggest that the divergence of major eukaryotic multicellular groups likely occurred in early Neopreterozoic or even earlier. Complex multicellularity also entered the fossil record during this time. The early Neoproterozoic strata in XuHuai (Xuzhou-Huaibei-Huainan) region, southeast North China Craton host abundant megafossils preserved as carbonaceous compressions. Some of them are very likely remnants of multicellular organisms and provide a unique perspective on the appearance of macroscopic life in the early Neoproterozoic and their evolutionary pathway to complex multicellular organisms. However, the biological affinity of these megafossils has been a matter of debate because of their morphological simplicity without described microstructures. Recent excavations have revealed a number of delicate specimens preserved defining microstructures, providing significant implications on their multicellular nature. We plan to carry out a more extensive excavation of megafossils from different horizons and sections in XuHuai region in order to obtain more specimens preserved inner structures. Our comprehensive investigation of those fossils will base on detailed characterization using stereomicroscopy, scanning electron microscopy, focused ion beam scanning electron microscopy, transmission electron microscopy and micro-CT. Our goal in this study is to evaluate and discuss the systematic paleontology, morphological complexity and phylogenetic affinities of those megafossils in early Neoproterozoic. This research will reveal the appearance of macroscopic life in the early Neoproterozoic, and provide essential fossil evidence for discussion of the origin and early evolution of multicellular organisms.

复杂多细胞生物的出现是生命进化史上的重大转折之一。它们从埃迪卡拉纪开始辐射，在显生宙主宰了地球生物圈。分子钟显示多细胞生物主要类群的起源可追溯到新元古代早期甚至更早，其化石证据也在该时期有所发现。我国华北地台徐淮地区新元古代早期地层，保存有大量宏体碳质压膜化石。其中部分很可能是多细胞生物化石，是了解多细胞生物在该时期面貌及其形态从简单到复杂的演化过程的宝贵窗口。由于形态简单、缺少确切的内部结构，这些化石的生物属性一直存在较大的争议。前期工作在该段地层中获得了保存多细胞结构的标本，为研究该类化石提供了新思路。本研究拟在不同剖面与层位，扩大化石挖掘规模，争取获得更多保存内部结构的标本。室内分析将结合实体镜、扫描电镜、聚焦离子束电镜、透射电镜和micro-CT等进行综合研究。厘定和探讨其系统古生物学、原始形态和生物属性等，为人们展现新元古代早期的宏体生物面貌，为探讨多细胞生物的起源与早期演化提供直接的化石证据。

多细胞真核生物的出现是地球生命进化史上极为重要的革新事件。分子钟显示，多细胞生物的主要类群（植物、动物和真菌）在新元古代早期（甚至更早）即已起源，然而该时期可靠的多细胞生物化石极为稀少。我国华北地台胶辽徐淮地区的新元古代早期地层，产出了大量碳质压膜宏体化石与微体化石，为我们探究生物多细胞化的过程提供了一个独特的窗口。本研究从徐淮地区刘老碑组、沟后组以及邻区鲁西地区土门群佟家庄组和石旺庄组、邻区辽南地区南芬组等新元古代早期地层中采集了大量保存精美、形态多样的宏体化石和微体化石标本。在淮南地区刘老碑组中报道了具异形胞蓝藻化石Anhuithrix magna，将最早可靠的具异形胞蓝藻化石记录由泥盆纪向前推进到新元古代早期，提供了生物固氮的最早化石证据。同时，在刘老碑组中发现具多细胞结构的化石Chuaria，提出Chuaria可能具有两个阶段的生活史：第一为底栖的休眠囊胞阶段；第二为营浮游生活的群体阶段。本研究揭示了刘老碑组中带刺疑源类化石Trachyhystrichosphaera aimika的膜壳具有复杂的三层式超微结构，并正式提出T. aimika可作为新元古代早期地层对比的标准化石。在淮北地区根据综合沉积岩石学、生物地层学和碎屑锆石年代学数据，对原始的沟后组进行修订，将“大不整合”置于修订后的沟后组和猴家山组之间，代表了~200-300 Ma的沉积间断。在邻区鲁西地区土门群中报道了多种宏体化石和微体化石。土门群的生物面貌可直接与徐淮地区新元古代早期的生物面貌相对比。本研究通过碎屑锆石年龄约束土门群佟家庄组沉积时代为新元古代早期（<1063 ± 53 Ma）。在邻区辽南地区南芬组中报道了一种新的多细胞绿藻化石Proterocladus antiquus n. sp.，复杂的形态特征表明其为冠群绿藻，将最早的多细胞绿藻化石往前推进了约2亿年。本研究为还原新元古代早期、雪球地球事件之前我国华北地台多细胞生物和其他共存的单细胞生物的整体面貌提供了直接的化石证据，也为徐淮地区与胶辽地区前寒武纪地层的归属和对比提供了可靠的生物地层学资料。