Systematics And Paleoecology Of Large Bivalved Arthropods From The Balang Formation(Cambrian),Guizhou,China

The large bivalved arthropods comprise an artificial group known mostly from two carapaces of Cambrian age.The bivalved condition in arthropods evolved convergently in various lineages.For many taxa,non-biomineralized anatomy is limited,and the term ‘large bivalved arthropod' is used until sufficient morphologic information becomes available to allow inclusion of taxa within recognized clades.Large bivalved arthropods are diverse,comprising many genera and species.They constitute an important assemblage of early,biomineralising metazoans,and collectively,are known worldwide from Cambrian strata.Most large bivalved arthropods were probably nektic or nektobenthic.Some have been interpreted as predators or more generalised carnivores,and some have been interpreted as sediment deposit-feeders.Incomplete morphologic information tends to hinder systematic work,and to render speculative the biomechanics and palaeoecology of many taxa.The Balang Biota from the Balang Formation(Cambrian Series 2 Stage 4)of Guizhou,China,contains representatives of 8 phyla.Of which,the arthropods contain many wellpersevered specimens such as trilobites,large bivalved arthropods,Anomalacaridid and so on.Based on the large bivalve arthropod materials from the Balang Formation(Cambrian Series 2 Stage 4)of Guizhou,China,and comparative fossils from Cambrian Strata(Series 2-3 Stages 3-5)of some other localities in south China,a detailed systematic study on their taxonomy,ecology,taphonomy and anatomy has been done.The large bivalve arthropod assemblage of the Balang Formation consists of 9 genera and 14 species,covered 2 new species: Isoxys jianheensis Liu et al.,2018,I.acutangulus(Walcott,1908),I.auritus(Jiang,1982),I.globulus Liu et al.,2018,Tuzoia lazizhaiensis sp.nov.,T.sinensis Pan,1957,T.bispinosa Yuan and Zhao,1999,Occacaris.lazizhaiensis Wen et Peng,2017,Branchicaris guizhouensis sp.nov.,Forfexicaris cf.F.valida Hou,1999,Canadaspis sp.,Pectocaris sp.,Combinivalvula sp.,Waptia sp.Of which,Isoxys and Tuzoia are much more abundant than any other genera,this is an important evolutionary character of large bivalve arthropods in Balang Biota.Lazizhai Section is the most productive section of Balang formation,the other sections exhibit a slightly lower abundance and lower diversity than the Lazizhai section,the other 7 genera are first reported in the Balang formation.The Large bivalve arthropods assemblage of the Balang Formation shows high biodiversity of the Balang Biota.Based on large collection of documents and data of previous and new studies,Cambrian Large bivalve arthropods assemblage characters and evolutionary trend were summarized in global main research areas.From Cambrian Series 2 to Series 3 of Burgess Shale-type Biota,the carapaces diversity of this group experienced an evolutionary process that from low to high,both in morphology and structure.And most species are affected by the environment so much,some regional species suggest that phylogeny of animals is closely related with ecosystem.Tuzoia lazizhaiensis sp.nov.and Isoxys jianheensis are known from abundant material in the Balang Biota(Cambrian Series 2,Stage 4).Based on 143 specimens of T.lazizhaiensis and 98 specimens of I.jianheensis from Balang formation,Guizhou Province,China,ontogenetic stages of these two species can be distinguished.These materials provide information for the study characteristics of ontogenesis of other large bivalved arthropods,and this information is helpful to taxonomy research.One specimen of T.lazizhaiensis sp.nov.shows only a stalked eye and antenna,one specimen of I.acutangulus shows one pair of great appendages and one specimen of O.lazizhaiensis shows the partial trunk information.These discoveries provide an important anatomical reference in this group and laid a foundation for further classification and provided an opportunity to study the taphonomy of Balang formation.Some intact individuals from the Balang Formation that carapaces attached with appendages or eye structure,may suggest an in situ preservation;while at some levels,multiple specimens preserved on empty shell and the trilobites on the same slab exhibit good sorting and certain orientation,suggesting occasional current transportation,concentration and alignment;in most cases,carapaces of large bivalved arthropods are disarticulated,multiple carapaces on slabs showing chaotic orientations without sorting,and preserved in dispersal,which indicates sporadic current disturbance within low energy background environment.The Balang Formation yields slender,sinuous,and branching ichnofossils associated with carapaces of large bivalved arthropods,the tracemakers,of speculative identity,are inferred to have been feeding on decaying soft parts of insides of carapace and on the microbal halos formed around decaying bodily remains.If so,this implies that the carapace was covered with a thin refractive organic layer.The feeding behavior of the trace-makers could have reduced the chances of preservation of soft parts of large bivalved arthropods.Suspected bite marks have been found on carapace of Isoxys,the position of these marks located on the lower parts,the similar mark shape of other arthropods such as trilobites caused by Anomalocaris,suggest that I.acutangulus had strong swimming skill to escape.Multiple specimens of Tuzoia or Isoxys were found be preserved on the same slab,Carcasses or moltted shells are not sure,but may suggest that at least some species are live in group(lifestyle),which is important for recover the ecosystem of Balang Biota.The discovery of diversified large bivalved arthropods not only adds new taxa to the Balang Biota but also provides new information for understanding their palaeoecology,evolution,and geographic distribution.