Study On The Macroalgae In The Late Neoproterozoic Of The Shennongjia And Huangling Area,South China

Macroalga,as an important part of early life,provides important information on the evolution of early life and environment.Exploring of macroalgae has been started since 1876 when the first piece of algal carbonaceous compression was found by Powell in Grand Canyon terranes of America.So far,Precambrian macroalgae have been widely reported over the world,among which China has the most continuous and abundant localities and has become one of the key areas to understand the evolution of macroalgae.Although globally macroalgal biotas' records show continuity to a certain level,there are still a few weak links and stratigraphic incompleteness of macroalgal fossils in some critical horizons.Following an overview of macroalgal fossils in the Proterozoic,macroalgae in both Paleoproterozoic and Mesoproterozoic Eras show low diversity and are abundant in simple forms such as spherical,ellipsoidal,cylindrical,and ribbon-like forms.Macroalgal assemblages in early Neoproterozoic(Tonian)Period continued to be dominated by simple forms.But macroalgae suddenly became diverse and abundant in the Ediacaran Period,representing important morphological innovations.However,macrofossil records of Cryogenian,the transitional interval between Tonian and Ediacaran,are currently very rare,leaving an important gap in the evolutionary continuity of macroalgae.Are there any macroalgal fossils undiscovered in Cryogenina strata? What about the macroalgal assemblage if they do exist? What about their diversity and abundance? What roles do they play in the macroalgal evolution? To address this knowledge gap,only fossils from the related strata can provide direct evidence.Another challenge in the study of Precambrian algae lies in phylogenetic and ecological interpretations.Biochemical and cytological data,which are traditionally used in the classification and phylogenetic analysis of modern algae,are not available in algal fossils.Some scholars try to study the morphological and ecological evolution of Proterozoic macroalgae through thallus morphology,which are still at the descriptive level.In the past decades,dozens of macroalgal fossils have been reported from Precambrian and chronologic studies provided more precise age restriction,making the statistic analysis of macroalgal succession pattern feasible.Meanwhile,some models or concepts(e.g.,morphogroups and functional-from groups)which have been widely used in ecological analysis of modern macroalgal communities,are also helpful in understanding the morphology and ecology of macroalgae during Proterozoic.Given the foregoing,I selected several critical horizons for fossil investigations,including the Cryogenian Nantuo Formation of the Shennongjia area,the early Ediacaran Doushantuo Formation and the late Ediacaran Miaohe Member of the Huangling area.Through systematically investigation,a great amount of macroalgal fossils were collected.Totally,29 genera and 43 species(including 6 undefined species)and 6 undetermined types are identified and described in this study.In addition,some synonyms fossils are discussed and revised.These fossils established the foundation for latter statistic analysis.1.Cryogenian Songluo biota: we investigated the Nantuo Formation in eastern Shennongjia area and discovered an assemblage of moderately diverse macrofossils,which contains at least 11 morphotypes and belongs to seven morphogroups(MGs)and four functional-form groups(FFGs).The Nantuo assemblage includes simple and longlasting forms such as discoidal,ovoidal,and ribbon-like forms,as well as complex forms with holdfast-stipe-blade differentiation,dichotomous branching and asymmetrical branching.It is interpreted as a transitional assemblage that share some simpler forms with pre-Cryogenian assemblages and some more complex ones with Ediacaran and Cambrian assemblages.Thus based on its abundance and particularity,we propose the nomination of “Songluo Biota”.The Songluo biota herein is a summation of all kinds of macrofossils from the Nantuo Formation in late Cryogenian(Marinoan ice age)with the representative of macroalgae.The discovery of Songluo biota is important for exploring fossil assemblages,environmental condition,and their relationships during the Marinoan glaciation.Besides,the Songluo biota tends to fill the gap between the Tonian and the Ediacaran macroalgae,which is vital to understand the evolution,replacement,and radiation of macroalgae.So far,the integrated macroalgal biota series from Paleoproterozoic to Neoproterozoic in China have been preliminary established.2.Phosphatized multicellular algae from early Ediacaran Doushantuo Formation in the Huangling area: well preserved phosphatized fossils have been reported in the Zhangcunping area,northern limb of the Huangling anticline,including Archaeophycus yunnanensis,Wengania globosa,W.exquisite,Sarcinophycus papilloformis,Thallophyca sp.,and ?Paramecia incognata.Combined with acritarchs found from the same horizon,the Zhangcunping fossil assemblage shows a distinct similarity with the lower biozone from the Doushantuo Formation in the East Yangtze Gorges area and the Weng'an biota in Guizhou Province,making it useful to refine the biostratigraphic correlation of the Ediacaran Doushantuo Formation in the Yangtze Craton of South China.More importantly,these phosphatized multicellular algae have cellular and organization structures,suggesting that thallus differentiation and morphological diversification of macroalgae have been occurred in the early Ediacaran Period.3.Macroalgae from the upper Ediacaran Miaohe Member in Shennongjia and Huangling areas: we report a large number of macroalgal fossils from the Miaohe Member at three new localities(Sanlihuang,Maxi,and Zhimaping sections)from the north and northwest of the Miaohe Village.These fossils mostly represent benthic multicellular algae and are taxonomically similar to those from the Miaohe Village,thus significantly extend the geographic distribution of the Miaohe Biota and provide new insights into the Ediacaran diversification of macroalgae.They are also helpful in understanding the paleoecology and taphonomy of Ediacaran macroalgal fossils.Combined with publised material and our fossil data,we firstly summarized the morphological evolution of macroalgae from Paleoproterozoic to Neoproterozoic Eras.To quantify the morphological evolution of macroalgae in the Proterozoic,we carried out a morphospace analysis of Proterozoic macroalgae based on Loduca et al.(2017).Nine morphogroups(MG-1~MG-9)and four functional-form groups(FFG-0~FFG-2.5)of Proterozoic macroalgae are identified.Two significant shifts in macroalgal morphospace occupation could be recognized in Cryogenian and Ediacaran,respectively.The Cryogenian shift is accompanied by the occurrence of new morphogroups(e.g.,the appearance of clavate,fan-shape,and branching forms),while the Ediacaran shift reflects the dramatically increase of some morphologically complex macroalgae(e.g.,clavate and dichotomous branching forms).In addition,the transition from discoidal holdfasts to rhizoidal holdfasts during late Neoproterozoic may be related to the decline of microbial mats in late Neoproterozoic.They may also reflect the initial establishment of the ecosystem with benthic macroalgae being the dominator.Based on analysis of taxonomic composition,morphogroups,functional-form groups and life habit,three floras of macroalgae in the Neoproterozoic Peroid are established : the Tonian Flora,the Cryogenina Flora and the Ediacaran Flora.The Tonian Flora was dominated by rather simple morphogroups(MG-1~MG-4)and functional-form groups(FFG-0~FFG-1),which shows indistinguishable difference with those from preNeoproterozoic periods.The Cryogenian Flora is represented by some innovative morphogroups(MG-2~MG-8)and FFG-1~FFG-2,whereas the Ediacaran Flora is mainly comprises of comparatively complex morphogroups(MG-1~MG-9).These significantly morphological and ecological changes of Precambrian macroalgae roughly concomitant with the atmospheric oxidation events and the environmental evolution.The late Neoproterozoic is one of the most critical period in the evolution of life and environment.The Cryogenian global glaciations followed by extreme greenhouse conditions are assumed to be a major ecological bottle neck to biological evolution,possibly leading to the emergence of new morphogroups of macroalgae during the late Cryogenian and the Ediacaran radiation in its aftermath.The term “the late Neoproterozoic macroalgal diversification” here is used to designate the diversified and widespread macroalgae during the late Neoproterozoic.Three major phases are recognized in terms of the timetable and taxonomic composition.The first phase is marked by the Cryogenian Songluo biota,which is dominated by simple discoidal and ribbon-like forms and also contains several firstly evolved forms such as branching form.The second phase occurred in the early Ediacaran period and represented by many fanshaped and dichotomous macroalgae,which also includes the appearances of phosphatized multicellular algae with well-preserved cellular and organization structures.The third and also the largest phase occurred in the late Ediacaran,which is marked by the radiation of different kinds of macroalgae with more complex morphology,more apparent tissue and organ differentiation,and more significant increase in abundance and diversity.Their characteristics are similar to modern algae.The late Neoproterozoic(at least the late Ediacaran)macroalgae form the basis of the Cambrian macroalgae.The holdfasts,branching thallus,and an upright habit of macroalgae reflected an active competition of environmental factors during late Neoproterozoic,while the evolution of macroalgae,in turn,promoted the environmental evolution and metazoan development,eventually leading to the Cambrian explosion.