Fossil Rhodophytes In The Ediacaran Weng’an Biota

The origin and early evolution of eukaryotic multicellular red algae(Rhodophytes)are amongst the most profound and mysterious questions in paleobiology and evolutionary biology.Currently,the widely accepted theory on the origin of the photosynthetic plastids of eukaryotic algae is endosymbiotic hypothesis,and it is also believed that the multicellularity of red algae arose independently in the Earth’s history.All these proposed hypotheses need to be tested rigorously by geological record.However,for a long time,the early fossil records of eukaryotic multicellular algae are scarce.Moreover,the preservation quality of these early fossil algae is generally low.Molecular clock estimates and biomarker evidence for origin and early evolution of multicellular red algae remain big uncertainty,leading a dilemma of theoretical testing.In this context,as a pre-Cambrian exceptional window with high fidelity,the Weng’an biota from South China plays a crucial role in reconstructing the early evolutionary process of red algae since so abundant materials interpreted as total group Rhodophyta have been reported previously.This study focuses on the multicellular red algae from the Weng’an biota,mainly investigating on the materials from thin sections of black facies and also some three-dimensionally preserved specimens from grey facies as complementary.An array of techniques including scanning electron microscopy(SEM),thin section digital scanning microscopy(tsDSM),confocal laser scanning microscopy(CLSM),synchrotron X-ray tomographic microscopy(sXRTM),confocal laser Raman micro-spectroscopy(CLRMS),have been used to characterize the morphological structures and in-situ mineral components of multicellular red algal fossils from different dimensions and scales(from two dimension to three dimension and from millimeter to submicron).We sort out the operating and working principles as well as the advantages of each technology in analysing phosphatized microfossils,and then developed a comprehensive experimental protocol based on the data we obtained.With the help of these techniques and based on the new observation of abundant materials,we found that the tetrads and octads observed in multicellular thalli are unlikely tetraspoes or octospores produced by a inferred free-living tetrasphorophyte phase of early stem group of corallinaleans,but are more likely to be precursors of berry-like "cell island" structures,which has been widely interpreted as carposporangia.We herein interpreted the tetrads and octads as the early developmental stages of "cell islands" and reconstructed the growth process of the "cell island" structures that they start by cell division from monad to dyad,tetrad and then octad cell packets,then gradually growing bigger by increasing the number of cells but not the volume of each cell.The large number of individually preserved cell islands discovered indicates that these structures may be released into environments as a whole for thalli reproduction,rather than producing carpospores.We also found for the first time a submillimeter scale but complete multicellular thallum with a rod-like shape and a disc-like holdfast.In the light of three-dimensional reconstruction aided by sXRTM,we has observed three cell layers with obvious cell differentiation as well as possible programed cell death process known as apoptosis,and more importantly,tetraspores embedded within the cortex layer near the surface.All the evidence combined as a suite suggests that this algal individual might be of sporophyte phase of a benthic rhodophyte.