Early Cambrian Small Shelly Fossils Along The Southern Margin Of The North China Platform

As a significant evolutionary innovation in the earth history,Cambrian Explosion created the majority of the existing animal phyla.However,for the lack of the basal Cambrian strata and well fossil records,Cambrian Explosion is still poorly known from North China Platform(NCP)as well as the age of its basal Cambrian strata and palaeogeographical position in early Cambrian.Small shelly fossils(SSFs),are one of the most critical fossil groups to research the Cambrian Explosion.Although,the SSFs had been widely discovered from the basal Cambrian strata along the southern margin of North China Platform,they are still poorly studyed and need more comprehensive researches to understand the whole faunal assemblage and their biostratigraphical and palaeobiogeographcial significances.Hence,we retake this work basing on the SSFs collected from the Xinji Formation at Chaijiawa,Shangzhangwan,Shuiyu and Sanjianfang sections and the Houjiashan Fonnation at Xiaomeiyao section along the Southern margin of North China Platform.An abundant SSFs fauna containing 47 genera and 54 species from the southern margin of North China Platform is described and illustrated herein.The molluscs contribute the highest diversity(?40%)to this fauna,comparing with hyolith(?20%),brachiopods(?15%),chancelloriids(?6%),sponge spicule(?2%),lobopodian(?2%),tommotiids(?2%)and other problematic fossils(8%).A diverse hyolith assemblages comprising ten genera(two new)and fourteen species(three new)are described including orthothecids(six genera and nine species)and hyolithids(four genera and five species).Most of these specimens are represented by both conchs and opercula.Some anomalous taxa possess characteristics of both Hyolithida and Orthothecida.The clavicles of Parakonrilithes mammillatus are documented for the first time which confirms its undoubted hyolithid affinity.Protomicrocornus and Ptaramicocornus may constitute a sister group of other typical hyolithids.The hyolith assemblages from diffrernt sections along the southern margin of NCP are coeval and can be correlated,with the Cambrian Stage 3 to lower Stage 4.The hyoliths from North China reveal closest compositional similarities to the faunas from eastern Gondwana and Laurentian.Seven genera,five species and two undetermined species of organophosphatic brachiopods are described basing on the material from the Xinji Formation at Shuiyu section.One new taxon,Paramickwitzia boreussinaensis,is erected which bears both of the characters of Setatella and Mickwitzia indicating its intermediate affinity between them.This brachiopod assemblage may range from Stage 3 to lower Stage 4 and reveals a remarkably strong biogeographical connection with the coeval faunas from East Gondwana,particularly South Australia,further indicating a close palaeogeographic position between North China and Australian East GondwanaIsolated plates of Microdictyon are discovered from North China for the first time,extending their palaeogeographical distribution in Cambrian.These plates bear some new characters,such as the single inclined platform-like apex and radial lines of the nodes.The palaeogeographical distribution and stratigraphical range of Microdictyon are reviewed.The low mushroom-shaped nodes could be a primitive and conservative character of Microdictyon,while the tall mushroom-shaped nodes may be derived.The first tommotiid taxon,P.pyramidalis,is documented in the Xinji Formation from North China Platform,extending the palaeogeographical range of this species outside Australia which further supports the planktotrophic development of the larvae of Paterimitra as a stem group brachiopod and indicates a close palaeogeographic position between North China and Australian East Gondwana during the early Cambrian.A diverse mollusca assemblage,consisting of 18 genera and 20 species,is recognized herein.This early Cambrian mollusca fauna from North China has the strongest bioprovincial connection with that of Australian East Gondwana,as well as somehow similarity with these from Eastern Laurentia and Siberia.Other taxa,including chancelloriids(3 genera and 4 species),sponge spicules(1 taxon),some problematic disarticulated sclerites(5 genera and 5 species)and some echinoderm ossicles are also described and illustrated herein.Amphigeisina?danica,Hippopharangites dailyi and Halkieria sp.are discovered from North China for the first time.Henaniodus nagicus is documented in the coeval strata from Australia for the first time?The SSFs assemblage from North China can be well correlated with that of many other continents during Cambrian Epoch 2,especially the middle-upper Dailyatiaodyssei Zone in East Gondwana.The age of this SSFs fauna could be middle Age 3 to early Age 4.The early Cambrian SSFs fauna along the southern margin of North China has significant palaeobiogeographic implication.This SSFs fauna bears relatively high similarity with East Gondwana,Luaretia,Siberia,Yangtze and Tarim.While,the highest SSFs faunal similarity shared among North China,South Australia and Antarctica suggests they may belong to the same bioprovince during the early Cambrian as well as the close palaeogeographical position of these three areas.