Early Cambrian Phosphatic-shelled Brachiopods From South China

The Cambrian “explosion” is a great event in Earth history when essentially all the animal phyla first appear in the fossil record,which provides a superexcellent window for exploring the early origination and evolution of metazoans.Brachiopods first appeared during the early Cambrian(Stage 2),and rapidly became one of the most important components of Palaeozoic marine communities.The excellent fossil records of the group highlight their diversity,abundance and long evolutionary history,which mark the brachiopods as one of the most important fossils in the fossil record.So,the study of the origination,diversity and evolutionary radiation of brachiopods will significantly improve our understanding of the mechanism,process and environmental effects of Cambrian explosion.The Yangtze platform of South China offers evidence within its Ediacaran–Cambrian geological record of the Cambrian explosion and diversification events in metazoan history.To understand the explosive radiation of animals and the environments in which it took place,the basal Cambrian fauna succession of the Yichang and Changyang in the Three Gorges area,the Fangxian in western Hubei Province,the Zhenba in southern Shaanxi province,and the Kunming in eastern Yunnan province have been studied.It reveals thousands of phosphatic shells,dominated by the earliest brachiopods and trilobites of South China.The shell morphology,internal anatomical structures,shell structures and ontogeny of the oldest known brachiopods have been well studied.The aim is to explore the early origination,radiation and differentiation of the early Cambrian brachiopods,to compare with the brachiopods preserved in shale of Yunnan province,and to reveal the different stratigraphic horizon and extension of different groups of brachiopods.Finally,a potential chronostratigraphic subdivision of the second series of the Cambrian,detailed studies of fossil assemblages and faunal successions on a regional or intercontinental scale and the intercontinental stratigraphic correlation of early Cambrian will come to a preliminary conclusion.It is well known that a variety of abundant brachiopods have been documented from the well-known Lower Cambrian Konservat-Lagerst?tten in eastern Yunnan province of southern China.These brachiopods represent one of the earliest-known examples of highly diversifiedmetazoan animal phyla during the Cambrian explosion.Nevertheless,these studies have been mainly focused on the description of specimens with exceptional preservation of soft parts,which is strictly controlled by the sedimentary environment and has a rare distribution.Apart from the soft tissues and organs inside,one of the most characteristic features that distinguish brachiopods from other metazoans is the possession of paired bilaterally symmetrical shell valves,usually with a larger ventral valve.In the Cambrian Konservat-Lagerst?tten the brachiopod shells are largely preserved as strongly clay-replaced imprints or moulds with no shell structures preserved.Interestingly,during the Early Cambrian phosphatization of Small Skeletal Fossils represents another specific taphonomic pathway that has high potential for excellent preservation of biological tissues,shell structure,subtle shelly fabrics and textures.A diverse fauna of lingulate brachiopods is described from the Early Cambrian of South China.Twelve species assigned to nine genera,belonging to five families and two orders,are systematically described.Among them,the taxa assigned to the Family Obolidae comprise Palaeobolus? liantuoensis,Spinobolus popovi,the Family Eobolidae comprise Eoobolus aff.viridis,Eoobolus sp.,Latusobolus xiaoyangbaensis gen.et sp.nov.,the Family Lingulallotretidae comprise Lingulellotreta ergalievi,Lingulellotreta malongensis.The Family Botsfordiidae is represented by Botsfordia minuta,Palaeothele xiadongensis gen.et sp.nov.,and the Family Acrotretidae is represented by Eohadrotreta zhenbaensis,Eohadrotreta? zhujiahensis,Palaeotreta shannanensis gen.et sp.nov.Herein,we present for the first time a detailed study of exquisitely preserved epithelial cell moulds and shell micro-structures in acid-etched material of the earliest acrotretid brachiopod Eohadrotreta zhenbaensis from the Shuijingtuo Formation of South China.The ventral and dorsal epithelial cell moulds(on average 18 ?m across)are usually isometrically convex or concave polygonal with a variable outline(tetragon,pentagon and hexagon).Investigation of the new material demonstrates that the disposition of pronounced epithelial cell pustules or depressions in shell interiors most likely is associated with regions of thickened muscle attachments in ventral and dorsal valves – a character that potentially could be of use in taxonomy,though detailed comparative studies of such epithelial microstructuresof acrotretids and other brachiopod taxa remain to be done.The earliest growth of post-metamorphic(post-larval)shells in two species of Eohadrotreta is first described from the Cambrian Shuijingtuo Formation of South China.Two different growth patterns can be observed by quantifying developmental variations in size and shape of successive stages of post-metamorphic shell growth(including the pedicle foramen forming stage,pedicle foramen enclosing stage and intertrough increasing stage)of Eohadrotreta zhenbaensis and Eohadrotreta? zhujiahensis.The pedicle foramen is never enclosed within metamorphic shell of E.zhenbaensis,while the enclosed pedicle foramen of E.? zhujiahensis is located directly outside metamorphic shell after the pedicle foramen enclosing stage.A strongly allometric growth pattern of E.zhenbaensis can be demonstrated by the early enclosure of pedicle foramen and an accelerated lengthening of the ventral intertrough is associated with the development of a more complex dorsal median septum during the intertrough increasing stage.By contrast,E.? zhujiahensis demonstrates a possible paedomorphoic development by a delayed enclosure of the pedicle foramen and an associated decreased lengthening of the ventral intertrough during the intertrough increasing stage.The ontogenetic development sequence represents the marginal accretionary forming and growth of the pedicle foramen,which resembles that of linguloid brachiopods.Furthermore,the development process of the pedicle foramen of Eohadrotreta seems to recapitulate the likely evolutionary transition from the Botsfordiidae with open delthyrium to the Acrotheloidea with an enclosed foramen.This study provides a unique opportunity to obtain a complete understanding of the ontogenetic development of the earliest acrotretoids,and casts new light on the phylogeny of lingulate brachiopods.Well preserved juvenile shells of the earliest Cambrian acrotretoid brachiopods Eohadrotreta zhenbaensis and Eohadrotreta? zhujiahensis,give new insights into early acrotretoid ontogeny with implications for elucidating the poorly understood early phylogeny of linguliform brachiopods.The acrotretoid larval body plan included a bivalved shell secreted at the beginning of the pelagic stage,as well as two pairs of dorsal setal sacs and a straight anterior–posterior gut alignment.These characters are also shared with both earlyCambrian mickwitziids,paterinates and lingulates as well as some later rhynchonelliforms(orthoideans and gonambonitoideans).This suggests that the acrotretoid type of ontogeny most likely is plesiomorphic for the Brachiopoda and probably was a character of stem group brachiopods.The acrotretoids were characterised by an indirect development,quite unlike recent lingulates,and heterochronic modifications clearly played an important role in the evolution of linguliform brachiopods.The hyolithid Paramicrocornus zhenbaensis from the lower Cambrian Shuijingtuo Formation of South China is well-preserved in three dimensions.The morphology of the conch and operculum of P.zhenbaensis shows that this species lacked helens,which are considered to be characteristic of hyolithids and hence Paramicrocornus may belong to a sister group of other hyolithids.The shell structure of P.zhenbaensis reveals close similarities to the shell structure of other hyolithids.Furthermore,the smaller size and non-radial orientation of tubules in the shell structure of the operculum also differ from that in orthothecid hyoliths,suggesting that this characteristic may be used to differentiate hyolithids and orthothecids.The phosphatized opercula of P.zhenbaensis exhibit a pair of muscle scars located close to the apex of the internal surface.These muscle scars,as well as similar structures in other hyolithids,probably served as attachment sites of muscles controlling the retraction of the tentaculate feeding organ recently discovered in hyolithids.Without helens,P.zhenbaensis may have been sessile with the conch partly buried in the sea floor.