Comparative Study Of Early Cambrian Brachiopods From Western Hubei And Eastern Yunnan

The Cambrian explosion has long been one of the key and heated-discussed topics with increasing interest in various fields of Geoscience.Major bilaterian clades abruptly appeared in the fossil record during the Ediacaran to Cambrian transitional interval,and then rapidly radiated in the early Cambrian,forming a marine ecosystem dominated by metazoans.Origination and evolutionary radiation of different phyla of metazoans are also the important,well-marked aspect of the Cambrian explosion.The Yangtze Platform of South China has well-developed Cambrian sedimentary successions,numerous exquisitely preserved fossil biotas,offering an unsurpassed opportunity to decipher the process of the origin and Cambrian explosion of metazoans.Brachiopods are among the most important faunal components of Palaeozoic marine communities,and have a long geological history dating back to the early Cambrian(Terreneuvian,Stage 2).Here,we focus on the brachiopods collected from the Cambrian Series 2 mudstone and siltstone of western Hubei and eastern Yunnan.The shell morphology,shell structure,biostratigraphy and paleoecology of brachiopods from South China have been well studied.The aim is to explore the origination and radiation of the early Cambrian brachiopods,so as to provide critical data on biostratigraphic correlation and faunal comparison of the lower Cambrian well developed eastern Yunnan and western Hubei provinces in the light of fossil brachiopods.Diverse and abundant brachiopods is described from the early Cambrian Shipai Formation in the Three Gorges area of Hubei Province,South China.Seven species assigned to six genera and seven families,including representatives of the subphylum Linguliformea: linguloids(Lingulellotreta ergalievi,Eoobolus malongensis,and Neobolidae gen.indet.sp.indet.),and an acrotretoid(Linnarssonia sapushanensis),and representatives from the subphylum Rhynchonelliformea: the calcareous-shelled Kutorginates(Kutorgina sinensis,Kutorgina sp.,and Nisusia liantuoensis).Acrotretoids(represented by Linnarssonia sapushanensis)are numerically abundant in the siliciclastic rocks from the Shipai Formation,and also constitute the dominant taxon(including Eohadrotreta zhenbaensis,Palaeotreta zhujiahensis)in the carbonate rocks from the Shuijingtuo Formation.In addition,the occurrence of the calcareous brachiopods Kutorgina and Nisusia in the Shipai Formation may represent the earliest records of this group in the Three Gorges area.In South China,brachiopods with the oldest trilobite Parabadiella first occur at the top of the lower Cambrian Shiyantou Formation,Meishucun section,eastern Yunnan.At this section,the siltstones of the uppermost Shiyantou Formation yield abundant fossils.The fauna consists of brachiopod,trilobite,bradoriid,chancelloriid,hyolithoid,lobopod and spheroidal fossils.As for the brachiopod,it just has two genera in this formation.To the middle part of the Yu'anshan Formation(Chengjiang biota),brachiopods are highly diverse and abundant.However,there are no shell beds of brachiopods from this level and most sessile benthic organisms in these deposits are attached to trilobite remains rather than to the shells of other brachiopods.Abundant brachiopod shell beds appear in the higher stratigraphic successions,including the Hongjingshao and Wulongqing formations.The mass occurrence of such bioclastic debris of organophosphatic brachiopods could have been a driving factor in the rapid diversification of stalked echinoderms(mainly eocrinoids),which are commonly found attached onto the shell valves of minute acrotretoid brachiopods,in,for instance,the Kaili Biota(Miaolingian,Wuliuan).The occurrence of remarkably rich brachiopod shell concentrations indicate that their shell valves began to serve as basibonts for other organisms to attach,and exert an important role in hardening and paving the soft-substrate seafloor during the early Cambrian evolution of Phanerozoic “mixgrounds”.The brachiopod fauna from the Shipai Formation is dominated by the acrotretoid Linnarssonia sapushanensis,which are commonly aggregated as patchy concentrations of shell valves on the same bedding plane.In contrast,the acrotretoids from the Wulongqing Formation form thicker shell beds(?11-13 pavements within a 1cm thick bed).Differential accumulation styles of acrotretoid valves highlight differences between sedimentary paleoenvironments and energy regimes of the Shipai and Wulongqing formations.The brachiopod assemblage and the first occurrence of Linnarssonia sapushanensis shell beds permit correlation of the Shipai Formation in the Three Gorges area of Hubei Province with the Stage 4 Wulongqing Formation in the Wuding area of eastern Yunnan.This correlation is further strengthened by the first appearance datum(FAD)of the rhynchonelliform brachiopod Nisusia in the upper silty mudstone of both the Shipai and Wulongqing formations.The new well-preserved material,derived from siliciclastic rocks,also gives critical new insights into the fine shell structure of L.sapushanensis.Microstructural studies on micromorphic acrotretoids(like Linnarssonia)have previously been restricted to fossils that were acid-etched from limestones.This is the first study to carry out detailed comparative ultrastructural studies on acrotretoid shells preserved in siliciclastic rocks.The primary layer forming the ornamentation of the external shell surface(e.g.,concentric fila)is preserved in some specimens of L.sapushanensis from the Shipai Formation.The secondary layer is also well developed,and has a columnar structure that is mainly composed of hollow tubes(diameter = 2.5 ?m on average),with solid columns(?1 ?m in diameter)that are composed of stacks of pinacoidal plates.This work reveals a hollow tube and solid column microstructure in the acrotretoid shells from the Shipai Formation,which is likely to be equivalent of traditional column and central canal observed in shells dissolved from limestones.