贵州清镇寒武系底部首次发现埃迪卡拉型盘状化石：生物学、埋藏学及其古地理意义

盘状化石是埃迪卡拉纪（Ediacaran, ≈震旦纪）最早被描述的生物分子,也是全球埃迪卡拉生物群最重要的组成部分,对于重建埃迪卡拉纪—寒武纪过渡时期的生态系统,了解“寒武纪大爆发”时期生物的早期演化至关重要。Aspidella是埃迪卡拉纪圆盘状化石的一个形态属,也是全球分布的埃迪卡拉纪沉积物中的典型化石。本文描述了在贵州清镇埃迪卡拉系—寒武系桃子冲组下部硅质岩中,首次被记录的埃迪卡拉纪疑似Aspidella盘状化石（cf. Aspidella）,表现为以同心圆环、中央凸起和凹痕为特征的圆盘,三维结构扫描观察到其明显的阶梯状圈层变化。我们倾向于认为桃子冲组cf. Aspidella作为底栖软躯体叶状生物的固着器,其内部沉积物是在海水正常沉积作用下形成的,并发现了含Fe、Al物质的固着器生物膜碎片。证实生物固着器官受到微生物席的影响优先保存,而茎干和复叶却消失不见,黏土和砂砾的差异压实作用导致死亡后的生物固着器的球壁上产生了阶梯状的轮廓。结合贵州清镇埃迪卡拉系—寒武系桃子冲组地层中出现的生物组合与华北、柴达木地块的埃迪卡拉纪生物地层的相似性,以及埃迪卡拉纪到寒武纪过渡时期的全球古地理格局,华南板块由中纬度持续地向华北、柴达木地块所处的低纬度区域漂移,环境的改变可能是导致埃迪卡拉纪晚期生物分子在华南寒武系地层中得到种群延续的诱导因素,并进一步扩展了经典埃迪卡拉型化石在时间和空间的分布。

First discovery of the Ediacaran- type discoid fossils at the bottom of Cambrian in Qingzhen, Guizhou: Biological, taphonomic and palaeogeographic implications

Discoid fossils are the earliest and the most important components in the Ediacaran biota. Aspidella, a disk- like Ediacaran form genus, is a typical fossil in Ediacaran sediments distributed worldwide. This article describes the Ediacaran biota element Aspidella disc- like fossil (cf. Aspidella) which is here first recorded in siliceous rocks at the bottom of the Taozichong Formation of the Ediacaran–Cambrian transitional strata in Qingzhen of Guizhou, and studies its biological and ecological characteristics, as well as its preservation mechanism in the Cambrian strata. This is crucial for the rebuilding of the ecosystem during the Ediacaran–Cambrian transition and for the understanding of the early evolution of organisms during the “Cambrian Explosion of life”.Methods: In this article, the 3D view image of fossils was taken using the Leica- DVM6 3D microscopy system. And the micromorphological observations and microchemical analysis of fossils were performed using a HITACHI- SU8010 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). The mineral composition of the fossil was analyzed by whole rock X- ray diffraction (XRD, PANalytical B. V.) and Raman spectroscopy (LabRAM HR Evolution).Results: Ediacaran biota element Aspidella disc- like fossil (cf. Aspidella) which is here first recorded in siliceous rocks at the bottom of the Taozichong Formation of the Ediacaran–Cambrian transitional strata in Qingzhen of Guizhou as a disc characterized by concentric ring, central bulge and indentation. A 3D image of the Aspidella fossil shows obvious step- like layer changes. In summary, according to the SEM—EDS analysis, combined with the in situ Raman spectroscopy and the XRD analysis, cf. Aspidella is the morphological remains of holdfasts of organisms on the seabed, rather than geological sedimentary phenomena such as solid fossils or gas escape structures. Its special ring structures are formed by the rapid filling of holdfasts by sand and clay minerals during the process of burial and mutation.Conclusions: We tend to think that cf. Aspidella of Taozichong Formation is the holdfast of benthic soft- body phylloids, and its internal sediments are formed by normal sedimentation of seawater. Fragments of holdfast biofilm containing Fe and Al substances have been found. It is confirmed that the holdfast was preferentially preserved by a microbial mat, while the stem and compound leaves disappeared, and differential compaction of clay and sand produced a stepped profile on the bulb wall of the holdfast after death. Based on the similarity between the bioassemblages in the Ediacaran–Cambrian Taozichong Formation of South China and the Ediacaran in the North China and Qaidam blocks, and the global paleogeographic pattern during the transition period from Ediacaran to Cambrian, the South China Plate have moved continuously from the middle latitudes to the low latitudes of the North China and Qaidam blocks. Environmental changes may have induced the population continuation of late Ediacaran biota elements in the South China Cambrian strata, and further expanding the temporal and spatial distribution of classic Ediacaran- type fossils in geological history.