Radiolaria And Coexisting Microfossils From The Early Cambrian Deep-water Chert In South China

Interval from the late Ediacaran to the early Cambrian was among the most rapid changing periods in the earth's history,during which most of the known animal phyla occurred accompanied by the presence of modern-style marine ecosystem.This event was thus named the Cambrian explosion.Due to its gravity for both biology and environment,the event was frequently and constantly studied,which led to the establishment of multistepped pattern of the evolution from the late Ediacaran to the early Cambrian.The grand diversification was herald by the Ediacaran fauna,whose attenuation was intimately followed the appearance of biomineralized skeletons in the earliest Cambrian.The latter was represented by the Meishucun Small Shelly Fauna.The radiation was then brought to the crest in the Stage 3 with the advent of diversified animals in multiple lineages,symbolized by the well-known Chengjiang Fauna.During the same time,South China was in a relatively stable tectonic construction,and preserved expansively exposed continuously sedimented strata that provides valuable materials for the research of Ediacaran and Cambrian evolution.Another feature was the preservation of water-depth gradient,including environments from shallow shelf to deep-water slope and basin.However,even though our knowledge about early life is significantly more advanced nowadays,it is somewhat unbalanced.For one,most of the evidence of organisms in the latest Ediacan and early Cambrian was from relatively shallow shelf environments including restricted basins,where most of the fossils were discovered.Despite that deepwater sediments of late Ediacaran to early Cambrian are widely exposed in South China,represented by the chert of Liuchapo/Laobao/Piyuancun formations,fossils from deepwater settings are still scarce.Most of the time,only algae and sponge spicules were reported from these areas.The Cambrian explosion was not only characterized by the emergence of diversified animals,but also the advent of extensive biomineralization.While Calcification and phosphorization was adopted by most of the skeletonized animals during the Phanerozoic,the only metazoan from early Cambrian that produce silicified skeleton was sponge.However,protists in modern ocean widely use silica for biomineralization.Among them,radiolarian have the history dated back the Cambrian,which might be part of the biomineralization event in the earliest Cambrian.However,our knowledge of their early evolution was still incomplete,which is partly due to the scarcity of their fossils during the early Cambrian.Furthermore,while chert was an important lithology for the preservation of radiolarian,fossils hosted in chert was seldomly found from the Cambrian.Most of them were reported from shallow sediments.In order to obtain more perspective of the organisms buried in deep-water settings and also earliest radiolairans,chert of the Liuchapo and Niujiaohe formations were investigated to extract and study the fossils.Radiolarians were obtained from both of the formations.The fossils from Liuchapo and Niujiaohe formations were from Terreneuvian and Series 2,respectively.And the ones preserved in the Liuchapo Formation could be the oldest radiolarians by far.It not only proves that chert in the early Cambrian can also host radiolarians,but also indicates that radiolarians,as plankton,already appeared in pelagic settings during the start of the Phanerozoic.Morphologically,the early radiolarians were featured by spherical and multilayered shells,and without spicule(s),which should be classified as Spumellaria.At the same time,other types of radiolarians were also recovered,some of which may have spicule elements in their skeleton.This suggests that radiolarians may already have high disparity during the earliest Cambrian.Judging by their construction,Archaeospicularia may not represent the form of primitive radiolarians.Instead,Spumellaria with spherical and layered skeleton may have much deeper history than previously believed.This was also supported by the results from molecular phylogeny.It estimates that Spumellaria diverged about 150 Myr earlier than the oldest fossil records.Molecular appearance of radiolarians might be deep in the Neoproterozoic.In the deep-water chert strata,fossils of Palaeopascichnus and Horodyskia were discovered from the lower and middle part of the Liuchapo Formation.But they were usually abundantly preserved.In contrast with the almost barren lower and middle parts,the upper part yielded not only radiolarians,but also acritarchs,multicellular algae,membrane structures,small shelly fossils,Megasphaera,and sponge spicules.The membrane structures were also named as sheet-like fossils,which were indicated to be related with early animals.It was the first time that Megasphaera reported form the early Cambrian.Although its affinity was still uncertain,some researchers believe it represents the embryos of early animals.They were widely reported from the Doushantuo Formation of Ediacaran.This discovery may temporally connect the possible embryos with the more extensively admitted embryos from the early Cambrian.The deep-water chert strata from Ediacaran to Cambrian were always devoid of evidence for stratigraphic correlation.The report of acritarchs and small shelly fossils will significantly help the biostratigraphic correlation.On the other hand,some small shelly fossils were indicated to be the grasping apparatuses of chaetognath.Their discovery from both shallow and deep-water settings suggest they are pelagic predator.For the organisms obtained from the chert strata,two stages can be discerned.The first was the late Ediacaran that contains only possible protozoa and algae.Fossils of the second stage in the earliest Cambrian are,in contrast,much more diversified.The fossils indicates the establishment of a much more complex ecosystem,comprised by the multicellular algae and acritarchs as primary producer,sponge as benthic animal,radiolarians and chaetognath represented by small shelly fossils as pelagic predator.Their advent would also bring significant impact for the environment.The appearance of radiolarians with silica biomineralization suggest their involvement into chert-forming process,and a shift from the abiotic or microbially controlled silica cycle to biosilicified eukaryotes dominated in the early Cambrian.They provide fossil evidence for the source of chert during this period.As zooplankton,radiolarians with other animals would accelerate the export and burial of organic carbon into sediments,which would remarkably enhance the oxygenation of water column.It provides valuable information of the organisms in deep-water chert during this critical interval that was almost absent from previous studies.