Shell Microstructures Of Cambrian Molluscs And Hyoliths From The Xinji Formation Of North China

One outstanding innovation in the evolutionary history of animal life is the advent of biomineralized skeletons during the Precambrian-Cambrian transition(551–518Ma).Those non-biomineralized ancestors rapidly acquired the capacity to produce a variety of biominerals to construct their initial skeletons,probably triggered by primary seawater chemical composition,and then evolved into highly organized skeletons with various complex hierarchical microstructures adjusting to high biological pressure(the biological “arm race”)by the explosive radiation of metazoans in the Cambrian.The Cambrian phosphatized Small Shelly Fossils(SSFs),comprising a diverse group of animals,represent the first appearance of biomineralized skeletons in Cambrian.Among them,biomineralized lophotrochozoans,including molluscs,brachiopods and some other enigmatic fossil groups,like tommotiids,halwaxiids,maikhanellids and hyoliths are conspicuous elements.Here,we investigated abundant exceptionally preserved molluscan and hyolith specimens from the well-known Small Shelly Fossil assemblage of the Cambrian Series 2 Xinji Formation of North China,to illuminate the general composition of the molluscan and hyolith fauna in North China;to evaluate the early biomineralization of the two groups by analyzing their skeletal microstructures and primary mineralogy;and further to investigate the origination of various clades of the lophotrochozoans in Cambrian and the precise position of hyoliths in lophotrochozoan trees.Phosphatization could replicate very fine morphological characters and microstructural details of primary skeletons.Specifically,we studied more than 5000 molluscan and 1500 hyolith specimens from North China.Results show that the diverse molluscan fauna contains 20 species,including one bivalve,three stem-group gastropods,and 16 helcionelloid species.Hyolith assemblage consists of nine genera,twelve species,and one undetermined taxon totally.Among them,four genera five species are orthothecids,three genera five species and one undetermined taxon are hyolithids,while the others have a combination of characters of both hyolithid opercula and orthothecid conchs.This diverse molluscan and hyolith assemblage shares a large number of species with contemporaneous faunas from Eastern Gondwana,particularly,South Australia and Antarctica.These high similarities not only confirms the age of the Formation as latest Age 3 or early Age 4 of the Cambrian Epoch 2,but also strongly support the hypothesis that the North China Block was situated close to Northeastern Gondwana,most likely juxtaposed with South Australia in the early Cambrian.External shells of mollusc Pelagiella madianensis,Anabarella australis and Pojetaia runnegari are exclusive aragonite that constitutes various type of microstructures with a complex hierarchical pattern,i.e.lamello-fibrillar,fibrous foliated,foliated aragonite,crossed foliated lamellar and polygonal tablet.Hyolith conchs and opercula consist of vertical penetrations,lamello-fibrillar,bidirectional aragonite folia,unidirectional aragonite folia,crossed foliated lamellar and conical speta.These new findings demonstrate that Cambrian molluscs and hyoliths could generate very complex shell microstructures under exquisite controls by the primary mantle,and evolved very rapidly in the early Cambrian.In addition,the striking skeletal similarities between Cambrian molluscs and hyoliths indicate an almost identical biomineralizing mechanism of the two groups in contrasting to lophophorates,such as brachiopods,tommotiids and tentaculitoids.Consequently,from the view of biomineralization,we argue that the deeply homologous shell microstructures in Cambrian hyoliths and molluscs strongly strengthen the phylogenetic links between the two groups,but obvisouly not closely related to brachiopods and other lophotrochozoans.