Taxonomy And Evolution Of The Cambrian Animal Chancelloriids

Based on detailed comparison and investigation on over 7100 chancelloriid sclerites from the small shelly faunas in Cambrian Stage 3 Xinji Formation of North China and Stage 2 to 3 carbonate-dominated rock units(including Mt Terrible Formation,Wirrapowie Limestone,Wilkawillina Limestone,Ajax Limestone and Mernmerna Formation)of South Australia,and 819 chancelloriid scleritomes from Stage 3 Chengjiang Biota of South China and Stage 4 Emu Bay Shale of South Australia,the morphology,taxonomy,biomineralization,biogeography,biostratigraphy,taphonomy,ecology and phylogeny of the Cambrian chancelloriid group are systematically discussed.Firstly,the chancelloriid specimens from Chengjiang,Emu Bay Shale and Xinji Formation are attributed to 4 genera and 9 species,including Chancelloria cf.eros,C.australilonga,Allonnia tripodophora,Al.phrixothrix,Al.erjiensis,Al.nuda,Archiasterella pentactina,Ar.tetraspina and Dimidia sp.Chancelloria australilonga represents the only exceptionallypreserved chancelloriid species in the Emu Bay Shale,as well as in East Gondwana.Dimidia sp.is a new type of scleritome in the Chengjiang Biota,which is characterized by denselyarranged two-rayed sclerites and inconspicuous apical tufts.In general,different forms of isolated chancelloriid sclerites originally associated within the scleritome of a single species are often present in rock samples in the same relative proportion as in the scleritome.Therefore,a regulative three-step scheme reconciled within the framework of the complete chancelloriid scleritome is proposed to distinguish isolated sclerites taxonomically and effectively:(1)categorizing sclerites into different series and forms based on their overall structure;(2)figuring out and comparing the proportions of various sclerite forms in different rock samples by numeric analysis;(3)integrating the proportional statistics with sclerite composition of the known scleritomes then positioning the sclerites a corresponding classification.Secondly,the microstructure of chancelloriid sclerites is well recognized and analyzed.The sclerites are usually composed of an inner aragonitic layer and an outer organic layer.The surface ornaments including tubercles and foramen-peripheral granules are selectively distributed in some species.The construction of the aragonitic layer is distinguished as a threeordered hierarchical system,i.e.,the third-order fibrous bundles,the second-order fibrous lamellae and the first-order fibrous aragonitic layer.An improved model outlining the process of sclerite formation and biomineralization is proposed,in which the epidermal system of chancelloriids is subdivided into three parts: epithelium,endocuticle and exocuticle.(1)The outer organic layer differentiated from the exocuticle forms an organic template.(2)Calcium cations released by epithelium precursor combine with bicarbonate anions from the environment on the inner surface of the organic template and precipitate as aragonite needles.(3)Aragonite needles grow and assemble along their c-axis forming fibrous bundles,the bundles then arrange in longitudinal,tangential or irregular patterns to form fibrous lamellae.(4)The fibrous lamellae construct the aragonitic layer by stacking up parallel to each other.Thirdly,documents of chancelloriids in all over the world are thoroughly reviewed and 64 significant fossil sites are summarized,in which 10 are Konservat-Lagerst?tten and 54 are skeletal faunas.Chancelloria,Archiasterella and Allonnia are common chancelloriid genera distributed in almost all continents,while Dimidia is restricted in South China,Gondwana and Laurentia;Eremactis only in East Gondwana and Laurentia;Nidelric and Cambrothyra only in South China.The brief global chancelloriid biostratigraphy indicates that this animal group was initiated in early Terreneuvian of Cambrian,flourished during the late Terreneuvian to late Miaolingian,declined from the late Miaolingian to early Furongian and totally went extinct after the Paibian Stage.Fourthly,the deopositional environments of fossil assemblages that preserved with chancelloriids are distinguished as peritidal zones characterized by sandy and silty limestone,shoal-and reef-related areas marked by bioclastic limestone,restricted marine zones dominated by bedded wackstone and lime mudstone,offshore and shelf margins featured with shale and siltstone,and so on.Therefore,chancelloriids are suggested to inhabit a broad spectrum of environments.The simple body structure of the chancelloriid group indicates a filter-feeding lifestyle.And this metazoan is probably able to expand and contract the body cavity periodically to complete the digestive cycle.Finally,a cladistic analysis is conducted by listing and comparing 62 characteristics of 11 animal taxa(5 of them are chancelloriid genera),and a maximum parsimony tree is obtained.In the phylogenetic tree,chancelloriids are as a monophyletic group of basal eumetazoan that close to the Cnidaria clade.The genus Cambrothyra,which possesses both single-rayed and multirayed sclerites in various forms,is suggested to be the ancestral group of chancelloriids.Chancelloria,Archiasterella and Allonnia constitute the brunch of Chancelloriidae,while Dimidia and Nidelric,are positioned in another brunch: the new family Nidelricidae.