Algal Fossils From The Early Cambrian Kuanchuanpu Biota In Southern Shaanxi Province,China

Eukaryotic multicellular algae are one of the important primary producers in the Earth’s ecosystem,and their origin and evolution on the early Earth have been extensively studied.The Ediacaran Doushantuo and the Cambrian Age 3–5(Wuliuan)are richly documented with algal fossils,while the Cambrian Fortunian(Cambrian Age 1)between these two periods is the opposite,making it difficult to understand the trophic structure of marine ecosystems during Cambrian Fortunian.The Kuanchuanpu biota(Cambrian Fortunian,ca.535 Ma)in southern Shaanxi Province of China yields various three-dimensionally preserved,phosphatized microsocpic fossils,such as animal embryos,cyanobacteria,and algae,which provides an exceptional window for investigating the origin and evolution of animal phyla in the early Cambrian ocean.However,the algal fossils therein are poorly investigated with only three articles describing and discussing the multicellular algae with tetrad cell structure,tubular algae with characteristic of "fragmentation" and bifurcated algae,respectively.In this paper,on the basis of abundant microfossils obtained by the method of chemical decomposition,the external morphology and internal structure of the fossil algae in the Kuanchuanpu biota were systematically studied by means of scanning electron microscope(SEM),synchrotron radiation X-ray tomographic microscopy(SRXTM)and micro-computed tomography(Micro-CT),and the following progresses were made:1)Five taxa of algae were reported in the Kuanchuanpu biota,including Calathophycus irregulatus gen.et sp.nov.,Lotomorpha bifurca gen.et sp.nov.,Bidentomorpha cylindra gen.et sp.nov.,and two unnamed algae(Unnamed alga A,Unnamed alga B).And their biological affinities were discussed.2)The differences in the preservation of algal cells,such as cell wall only,cell wall and intracellular inclusions,without wall and intracellular,in the Kuanchuanpu biota were observed.Moreover,their taphonomical processes were speculated.In addition,this paper discusses the influence of algae on shallow-sea ecosystem during the transition period of Ediacaran to early Cambrian,from the perspective of the oxygen content,water quality and co-evolution with animals.The ecological and environmental effects of algae would drive and accelerate the explosion of animals.Subsequently,a complex modern-type shallow-sea ecosystem was built in the Cambrian Age 3.The discovery of five taxa of algal fossils obtained in the Kuanchanpu biota contributes to the diversity of algae in the beginning of the Cambrian.It also proves that eukaryotic algae have broken the incumbency of phototrophic cyanobacteria and promoted the modernization of shallow-sea ecosystem during this period.