Comparative Study And Ecological Environment Reconstruction Of The Ediacaran Macroalgae Gesinella And Baculiphyca

After the“snowball event”,the Ediacaran macroalga in South China,including the Wenghui biota in northeastern Guizhou,was reported globally.The Ediacaran biota in South China is dominated by benthic macroalgae,among which Gesinella and Baculiphyca are the advantaged and abundant macroalgae.Gesinella and Baculiphyca in Wenghui biota are found in the black shales of the Ediacaran（585.7±2.8 Ma-550.1±0.6 Ma）Doushantuo Formation in the Yangtze Platform of South China in the form of carbonaceous compression.Morphologically,Gesinella and Baculiphyca have similar forms that are not easy to recognize in the field,mainly reflected in that both are composed of the holdfast in the sediment and the unbranching thallus above the sediment.The holdfast can be divided into rhizome and rhizoids,and the thallus includes the upper lamina and the lower stipe.It shows that they have obvious organ differentiations and are high-level macroalgae metaphytes.However,there are significant differences in morphology between Gesinella and Baculiphyca.The body of Gesinella is usually narrow at the bottom and wide at the top（oval or oblong thallus）.The top part of the upper lamina is rounded or circular.There are circular spots on the surface of the thallus in individual specimens.The stipe is short-rodlike with large size at the top and small size at the bottom,and gradually transits to the upper lamina.However,the minimum width of the stipe is10-20 times narrower than the maximum width of the lamina.We suggest,in here,that the boundary between the lamina and the stipe is placed in a obvious distinction,an rapidly increasing width.A large number of branching or unbranching filament structures are regularly distributed on the surface of the thallus,meaning that the thallus may be a sac-like body.These recognizable filament structures generally grow from the stipe and extend upwards to the middle and upper part of the thallus,even near the top.It is speculated that the filament structures may act as a framework and resist the water column on the sac-like body of the thallus,while the filament structures have a flexible to strengthen and support strength the stipe.In addition,the stipe connecting with the lamina is three-dimensionally preserved as a cylindrical form with some obvious folds,which may have the function of reinforcing the sac-like body.The holdfast is obviously complicated and differentiated,usually composed of a cone-shaped rhizome and numerous filamentous rhizoids.It can be speculated that it is an important organ for serving to anchor the sac-like body in the sedimentary grains of the seafloor,suspending in the water column for photosynthesis.Alternatively,the thallus of Baculiphyca is generally in the shape of a long longitudinal rod,often like a baseball rod,characterized by a narrow and slightly wide lower part,smooth edges,often broken,without the preservation of the top part of the upper lamina,or difficult to identify due to incomplete preservation.Filament structures and other decorative structures are not observed on the surface of the thallus.The stipe of Baculiphyca is mostly preserved as a cylindrical form,and the upward diameter is gradually increasing or nearly equal to the smallest dispersion angle thickening.The top of the preserved fossil is common lamina-like features.A large quantity of filamentous rhizoids grow above the rhizome with thickened carbonaceous ridges,indicating that its"rhizoid system"is relatively developed.In this study,the width（diameter）and height（length）data of 108 samples of Gesinella and 103 samples of Baculiphyca are measured,statistically analyzed.The growth stages of Gesinella and Baculiphyca can be roughly divided into three stages,the juvenile stage（difficultly identified）,the adult stage in which the organism may grow mainly upward in height to compete sunlight for photosynthesis,and the organism of the senescent stage grows continuously in height and width for competing niche.However,there are significant differences in the maximum diameter（width）of the stages.Among the stages,the maximum width of the thallus in the juvenile stage of Gesinella is<3 mm（unidentified）,the adult stage is 3-10 mm maximum width of the thallus,and the senescent stage（growth fatigue stage）is>10 mm maximum width of the thallus.The maximum width of the thallus corresponding to the three growth stages of the Baculiphyca is:<0.5 mm（unidentified）,0.5-3 mm and>3mm.This study also focused on the research and speculation of the microstructure,composition and taphonomic characteristics of the Gesinella and Baculiphyca fossils,and selected some well-preserved samples for the technical analysis of scanning electron microscopy（SEM）,energy spectrum analysis（EDS）,Raman spectrum analysis（Raman）and two-dimensional Raman imaging.Among them,the possible differentiation of epidermis,cortex and pith tissue can be observed under the scanning electron microscope in the lower part of the thallus（or stipe）,which can correspond to the epidermis,cortex and pith tissue of higher plants.Nevertheless,there is no obvious difference between the two in the material composition of the energy spectrum analysis results.Because Raman spectrum shows the typical characteristics of disordered carbonaceous materials,organic carbonaceous materials generally have two distinct bands near 1350 cm^-1and 1600 cm^-1.In this study,a 1000 cm^-1to 1750cm^-1narrow spectral window was used to observe the Raman spectral characteristics of each component of the selected samples（4 samples）.Within the window range,the Raman spectrum characteristics of macroalgae Gesinella and Baculiphyca are mainly displayed in two distinct bands of～1350 cm^-1and～1600 cm^-1,indicating that these two macroalgae fossils are composed of organic carbonaceous materials.In this study,the Raman peak spectral intensity ratio（I-1350/1600）of 1350 cm^-1and 1600 cm^-1is used to analyze and compare the structure and organization/arrangement of organic carbonaceous materials in Gesinella and Baculiphyca.The analysis results shows that the carbonaceous structure and organization/arrangement of both in their holdfast,stipe and lamina are weak as a whole,and the I-1350/1600 value of each component part changed in a small range.At the same time,this paper also uses two-dimensional Raman imaging analysis to show the ultrastructural heterogeneity of Gesinella and Baculiphyca.The experimental analysis results show that there are significant differences in the abundance of organic matter in the holdfast,stipe and lamina of the two kinds of algae,and the high abundance areas present different forms.In the black carbonaceous shale of the Doushantuo Formation in northeastern Guizhou,Gesinella and Baculiphyca organisms are common in forming a dense layer of fossils.Fossils have preserved complete holdfast or rhizoids,indicating that they are mainly the benthic and fixed lives.And the holdfast and thallus are completely preserved on the same layer,indicating that they are buried in situ or near in-situ.Simultaneously,there are a considerable number of biological fossils preserved after bending or being broken,and there are specimens in directional arrangement,which means that their living environment is a marine environment with certain hydrodynamic conditions.In the Wenghui biota of the Ediacaran Doushantuo period,there are not only metaphytas that demand sunlight and have a competitive trend,but also a large number of active feeding metazoans that have developed digestive systems.It shows that high-level and relatively complex ecosystems have also been formed in the study area during this period.In addition,the oxygen content and salinity of the water in the Yangtze Sea during the Doushantuo period have reached the levels and conditions required for biological life and reproduction.Therefore,it is inferred that Gesinella and Baculiphyca should live in a shallow and low-energy environment with warm and euphotic zone,normal salinity,certain hydrodynamic conditions and certain oxygen content.Through the comparative study and ecological recontraction of the macroalgae Gesinella and Baculiphyca in the Ediacaran Wenghui biota,this paper provides useful data for the study of the morphology and ecological environment changes of this kind macroalgae living in the Ediacaran Doushantuo period,and offers important evidences and materials for the biological succession,marine environment changes and stratigraphic correlation research during this period.