| Numerous trace fossils emerged in the late Ediacaran Period that marked the enhancement of animal activities as well as provides important informations on our understanding of the Cambrian Explosion during the Precambrian-Cambrian boundary.Recent studies,especially emphasized the taphonomic evidences,many Ediacaran ‘trace fossils’ have been reinterpreted as body fossils or problematic fossils.Shaanxilithes ningqiangensis,one of those Ediacaran problematic fossils,is well-known by its wrinkled ribbon appearance,with 0.3-14 mm in width,and up to 18 cm in length.Shaanxilithes was first described from the upper Ediacaran Gaojiashan Member of the Dengying Formation,southern Shaanxi Province,South China.Subsequently,this taxon has been reported from many different geotectonic units of China,include the Yangtze Platform,North China Craton and Chaidam Block,expanded the palaeogeographic distribution of this fossil in rocks also of late Ediacaran age.Recently,Shaanxilithes has been also reported from the late Ediacaran horizons outside of China,e.g.Siberia,Namibia and India,greatly extended its biogeographical distribution in interregional scales.Shaanxilithes that combined narrow stratigraphic range with a cosmopolitan paleogeographic occurrences,making it a potential index fossil for upper Ediacaran successions in both regional and interregional scales,despite the phylogenetic affinities and body construction of this fossil remains unresolved.Shaanxilithes is usually preserved in siliciclastic rocks as a highly flattened ribbon and consequently,three-dimensional morphology and taxonomy of this fossil has long been enigmatic,with interpretations ranging from pogonophoran,sedimentary structures,trace fossils,algae to metazoans of uncertain phylogeny have been suggested.Meanwhile,various types of ribbon-shaped ‘trace fossils’ and ‘problematic’ fossils,include Shaanxilithes ningqiangensis,Shaanxilithes erodus,Helanoichnus,Palaeopascichnus,generally co-occur in the same layer of late Ediacaran rocks have been obtained in many continents.Recent to emphasize taphonomic evidences and,consequently,those ribbon-shaped fossils assembles described above have been considered synonyms of Shaanxilithes ningqiangensis,despite no confirm evidence to testify this speculation.Our specimens described herein were collected from seven distrinct regions of China,include the Helan mountain area of Ningxia Hui Autonomous Region,Lushan area of Henan Province,Luonan,Ningqiang and Longxian areas of Shaanxi Province,Quanji mountain area of Qinghai Province,Jinning area of Yunnan Province.Here,new materials reveal that Shaanxilithes ningqiangensis is an internally nested tubes encased with a membranous,annulated external tube,rather than a ribbon-shaped trace fossil as some previous researchers have suggested.Meanwhile,new materials reported here also provide a direct evidence to testify previous speculation that above discussed morphological types,including Shaanxilithes ningqiangensis-type,Helanoichnus-type,Shaanxilithes erodus-type and Palaeopascichnus-type,can be observed in different parts of a single specimen.Moreover,numerous morphological statistics also provide a indirect evidence for discussing their potential relationship that the morphological differences of ribbon-shaped fossils described above have been suggested largely based on taphonomic variations.Morphological reconstruction of Shaanxilithes suggest highly comparability of the animal with modern tube worm Siboglinidae(Polychaetea,Annelida).Integrated datas of morphology,petrology and statistics suggest that Shaanxilithes exhibited a sedentary,epibenthic life-mode,with its tail anchoring to sandy or muddy substrate.The appearance of tubular bodyplan in late Ediacaran period might be a consequence of self-protection of organisms,which also marked the arising of the earliest predator and construction of the earliest ecosystem.Shaanxilithes represents one of the earliest non-biomineralized tubular organisms,is of significant innovation in the evolutionary history of life during the Precambrian-Cambrian transition.In addition,our reconstruction shows that the bodyplan of Shaanxilithes is analogous to Cloudina as well,a well-known biomineralized tubular fossil in upper Ediacaran strata.The evolutionary trend from non-biomineralized Shaanxilithes to biomineralized Cloudina possibly indicate the increased predatory pressure and biological activities,and thus provide important informations for understanding the advent of biomineralization in early animals.Moreover,the biological arms might triggered the radiation of early animals during the period of Cambrian explosion.The Ediacaran fossils are significant in terms of preservation.Here,our datas reveal that authigenic clay association played a pivotal role during the initial preservation of Shaanxilithes,and then underwent a series of transformation in subsequent diagenesis and weathering processes.On the western-southwestern margins of North China Craton(NCC)and northern margin of Quidam Block,Shaanxilithes-bearing horizons always rest conformably on the Neoproterozoic glacial deposits,and are disconformably overlaid by the Cambrian(Stage 3-4)phosphorites.Specifically,the new finding of Shaanxilithes from the upper Ediacaran Dongpo Formation along the southwestern margin of the NCC greatly extended its palaeogeographic distribution as well as provide convincing evidence for determining the age of the Dongpo Formation and underlying Neoproterozoic glaciation Luoquan Formation.Many geotectonic units in North China,include the northern margin of the NCC,Alashan massif,Xingxingxia-hanshan massif,preserve a consistent stratigraphic sequences through the Precambrian-Cambrian boundary.From this perspective,we re-evaluate the potential Shaanxilithes-bearing horizons in those geotectonic units as mention above. |
