Geobiology Of Low-trophic-level Functional Groups In Marine Ecosystems After The End-permian Mass Extinction In South China

The Permian-Triassic(P-Tr)transition was a critical time in the evolution of life on Earth.Asthe largest mass extinction event since the Phanerozoic,the end Permian extinction greatly reshaped the evolution trajectory of ecosystems.The transformation of ecosystem structure,the evolution of organisms,the interaction mechanism between organisms and the environment,and the recovery parttern during this special period have long been the research hotspots of geoscience.After the extinction event,although different species recovered at different times,thecomplex marine ecosystem did not recover until eight million years later,in the Middle Triassic.Many studies showed that the marine ecosystem gradually rebuilt from the bottom to the top after the extinction,showing a step recovery parttern,that is,the low-trophic-level functional groups(i.e.primary producers)recovered first,followed by primary consumers,intermediate consumers;In contrast,the high-level consumer community at the top of the trophic pyramid was the last to recover.Therefore,the low-trophic-level function groups are an indispensable part of the whole ecosystem recovery process.In particular,the interaction between primary consumers and primary producers is an important driving force of the step-by-step recovery process of ecosystem,and the efficiency of their work directly affects the rate of ecosystem recovery.In earliest Triassic,marine ecosystems were dominated by primary producers and opportunistic consumers.This period represents the shambles after the P-Tr mass extinction,which is not only the paradise for post-extinction opportunist taxa,but also the hotbed for biological recovery.The research of this period can help to explore the survival principle of the opportunist taxa,explore the biological trigger at the beginning of recovery,as well as reveal the complex relationship between lives and lives and between lives and environment after the great dying.Therefore,this study attempts to comprehensively study the biological characteristics of the low-trophic-level groups in marine ecosystem recorded in several sections representing different sedimentary facies(or sedimentary environments)of the Lower Triassic in South China,focus on the widespread micobialites in shollow marine facies and low-trophic-level groups in deep water basin facies,reveal the biological features of low-trophic-level groups after the mass extinction,and discusses the interaction between lives and the importance of their effect on promoting the reconstruction of the ecosystem recovery.This study will contribute to the discussion of the mechanism of ecosystem reconstruction.Here six sections from South China were selected for detailed observation and study of the microorganisms in microbialites,including Xiushui,Xiejiacao,Dongwan and Laolongdong sections in the middle and lower Yangtze platform,Yudongzi section in the upper Yangtze region and Zuodeng section in the Nanpanjiang Basin.This study shows that thesemicrobialites generally developed a special microbial fossil.Its morphological and ecological characteristics can be compared with that of Jurassic Gakhumella.These microfossils are widely distributed and abundant in the microbiolites,indicating that they may play an important role in the formation of the microbiolites.Study of huge number of individuals show that these microorganisms undergo a four-stage reproductive process,which is exactly the same as that of Pleurocapsales of modern cyanobacteria.Electron microscope and energy dispersive spectroscopy analysisshowed that Gakhumella were closely related to Mg enrichment.Comprehensive studies showed that this microorganism played an important role in the calcification,construction and dolomitization of microbialites.The Permian-Triassic boundary microbiolites are rich in calcareous globular microfossils.One type of fossil with a spiral shape was discovered for the first time.Their morphology and structure are very consistent with that of early calcareous nannofossils,so the very abundant calcareous nannofossils in the Meso-Cenozoic may have originated in microbialite ecosystems in shallow water carbonate platforms after the end-Permian mass extinction.Thus,this study will bring forward the origin of accepted calcareous nannofossils.And,the end-Permian extinction not only destroyed the upper part of trophic pyramid,but also changed the lower part of trophic pyramid,gave birth to a new type primary producer,heralding the evolution of a whole new group of organisms that represent the upper part of trophic pyramid.These microbiolites also contain keratose sponges,which have been reported from contemporaneous microbialites in Armenia,central Iran and Turkey,and for the first time from microbialite profiles in South China.The microbialites in Xiushui,Laolongdong,Dongwan sections are characterized by keratose sponge fibers,and their aggregates form a maze-like network.The contemporaneous occurrence of keratose sponges in South China,Armenia,central Iran,and Turkey(covering eastern and southern Paleotethys,and western Neo-Tethys)suggests that the sponges were widespread after the P-Tr mass extinction and played an important role in the construction of metazoan-microbial reefs.This thesis also analyses the time and space distribution pattern of the keratosa sponge,the results show that the appearance of keratose sponge,on the whole,coincide with the peak of the abundance of microbialites,and seems to be particularly rich and widely distributed in Cambrian,post-Late Devonian extinction and post end-Permian mass extinction.Keratose sponges are mainly distributed in tropical regions,but also in temperate regions,which is consistent with the environmental preferences of modern species,indicating that they are obviously conservative in evolution and environmental distribution.After the end-Permian mass extinction event,keratose sponges appeared not only along with microbialites in shallow marine,but also in deeper basin facies.The fossil sponges were found in mudstones at the bottom of the Lower Triassic in Kejiao section,Huishui County,Guizhou Province,about 4.5m to 6.5m above the Permian-Triassic boundary.These sponge fossils are mostly preserved as round to long elliptic black impressions with a diameter of about 1 cm.They are abundant and,once present,spread throughout the whole layer.Most of the specimens preserved clear pores and central osculum structure,some of the themhave lateral buds There are thin-shell brachiopods and ammonites in symbiosis with these sponges.Based on morphological study,microscopic observations and comparing with modern sponges,these fossils can be classified as Keratosa in Demospongiae,and it is highly similar to modern genus Smenospongia and may have a genetic evolutionary relationship,thus erected a new genus Palaeosmenospongia gen.nov.Analysis of the morphology and size of framboid pyrite and trace elements in the Permian-Triassic transition strata of Kejiao section shows that these keratose sponges were preserved in anoxic environment.In this study,keratose sponges were found in different regions and water depths after the end-Permian mass extinction,indicating that the sponges were highly adaptable and widely distributed in the post-extinction marine.There is growing evidence show that massive ocean anoxia event was the most direct killer of most life during the end-Permian extinction.The ocean in earliest Triassic usually inherits an anoxic environment,act as desert and grave for most life.However,studies of modern sponges have shown that sponge animals have highly anoxia tolerance.And the symbiotic cyanobacteria in sponges could produce large amounts of oxygen and nutrients.Moreover,sponges can convert dissolved marine organic matter into particulate matter,which can be used at higher nutrient levels.Thus,even in extremely anoxic and harsh environments,high diversity of ecosystems can develop from the sponge-microbial community.This study proposed that the microorganisms together with keratose sponges from eariliest Triassic microbialites ecosystem and deep marine anoxia environment function as an ecological engine in the harsh post-extinction marine.The sponge-microbial community is not only a symbol of the harsh post-disaster environment,but also promotes the recovery and reconstruction of the lower part of ecosystem pyramid.It is the first step in the long process of ecosystem recovery after a great dying and the initial driving force of ecosystem pyramid recovery and reconstruction.