Taxonomy And Ontogeny Of Bivalved Arthropods From The Lower Member Of The Shuijingtuo Formation,Series 2 Stage 3,Eastern Three Georges Area,South China

Bivalved arthropods were widely distributed in the Cambrian ocean and an important component of the arthropods in the Burgess Shale-type Lagerst?tten.In this study,based on over 260 specimens,two bivalved arthropods,Caudicaella bispinata(Cui and Huo,1990)comb.nov.and Sunella Grandis Huo,1965,were described from the Cambrian Shuijingtuo Formation,Series 2 and Stage 3,South China.Both genera are assigned to the emended family Sunellidae Huo,1965.C.bispinata is characterized by elongated cardinal spines,semicircular valves with dilated doublure and a distinctive anterodorsal sulcus.Sunellidae was revised,with newly identified features such as an anterodorsal sulcus extending from the middle of the anterior spine to the anteromedian part of the carapaces.Statistic and ontogenetic analyses indicate the valves of both genera grow isometrically,while cardinal spines of C.bispinata grow allometrically.Analyses using EPMA-WDS has revealed that fossils from the lower Member of the Shuijintuo Formation in Changyang area were preserved as pristine carbonaceous film.Functional analogy suggests a pelagic lifestyle of C.bispinata.In addition,the palaeogeographic distribution model of C.bispinata is helpful for a global biostratigraphical correlation in the Cambrian Period.