Taphonomy Of The Cambrian Kuanchuanpu Biota And Developmental Biology Of Olivooids

The early Cambrian Kuanchuanpu Formation in south Shaanxi Province yields not only abundant phosphatized small shelly fossils,but also various soft-bodied animals at embryonic and adult stages with three-dimensional preservation,providing a crucial taphonomic window to explore the diversity of biomineralization of early metazoans,the early evolution of development of metazoans as well as the early evolutionary radiation process of metazoan body plans.Olivooids including developmental stages of eggs,embryos and hatchlings are iconic fossils in the Kuanchuanpu Biota and the investigation on their developmental biology and phylogenetic affinities has always been a frontier hot topic in the field of the Cambrian explosion.More and more new fossil materials of olivooids have been discovered in the past years,however,the ontogeny and life cycle as well as the phylogenetic position of olivooids remain controversial.This controversy is at least in part due to the difficulty in reaching a consensus on the biological interpretation of the anatomical structures of olivooids during different developmental stages.Furthermore,there is also a lack of discussion on the sedimentary background,taphonomic process,and preservation patterns of the Kuanchuanpu fossils,which hampers distinguishing biological structures from diagenetic artifacts to a certain extent,and ultimately intensifies the debate.To figure out the phylogenetic affinities of olivooids from the Kuanchuanpu biota,resolve the current disputes,and then fully explore the potential evo-devo biological significance of olivooids,we perform the research from two aspects of taphonomy and palaeonbiology.(1)For taphonomy,to explore the sedimentary environment,taphonomic process and preservation pattern,and identify the effects of burial mechanisms on the primary structures of organisms,we targeted the host rocks of the Kuanchuanpu biota,phosphatic limestones from the Kuanchuanpu Formation in Dahe Town,Xixiang County,Shaanxi Province,and conducted comprehensive microscopic analyses based on confocal laser scanning microscopy,scanning electron microscopy and Micro-CT,and spectroscopic analyses based on X-ray fluorescent and confocal laser Raman micro-spectroscopy on the microtubular fossils Conotheca and the rock matrix.The results included information of spatial distribution of elements and minerals within microtubular fossils and rock matrix,spatial distribution of microtubular fossils within the rock,as well as microstructures of the microtubular fossils.We analyzed the sedimentary characteristics of tubular fossils at the scale of hand specimens,and discuss the sedimentary process of Kuanchuanpu Biota on this basis.By analyzing the internal structure of fossils,it is concluded that the multi-layer nested structure of tubular fossils is a random nesting phenomenon generated during the deposition process.We identified some secondary diagenetic structure of the tubular fossils,like linings,partly separate and multi-nested structure of tubes.Therefore,it provides new insights not only into the biology of these microtubular fossils,but also the preservation mechanism of the Kuanchuanpu Biota.(2)For paleobiology,we discovered a new type of olivooid fossils at different developmental stages and reconstructed their internal three-dimensional soft-bodied structures using synchrotron radiation X-ray microtomography and high-resolution micro-CT.Based on the new anatomical structures,we tested various competing hypotheses and proposed that olivooids are more closely related to scyphozoan jellyfish rather than cubozoan jellyfish.Hypotheses about echinoderm or cycloneuralian affinities of olivooids proposed previously can be ruled out.Our discovery of new fossil materials of olivooids provides new insights for understanding the development of early Cambrian cnidarians.