Palaeobiological Study Of Hyoliths From The Cambrian Series 2 Lagerst?tten In Eastern Yunnan,China

Hyoliths are extinct benthic invertebrates with calcareous shells that range throughout the Palaeozoic.As one of the earliest metazoans with biomineralized skeletons,hyoliths are abundant,diverse and widespread in the Cambrian,providing an excellent topic for understanding the Cambrian explosion and the mechanism of biomineralization.Despite hyoliths having been studied for nearly 200 years,their biological affinity remains controversial.Hyoliths are generally separated as two orders/groups Orthothecida and Hyolithida,due to morphological distinctions in the operculum and conch and the existence of helens.However,the limited understanding of the group in earlier palaeontological research together with poor preservation has led to uncertainties in the taxonomy and systematic descriptions of many Cambrian hyoliths that were initally based on fragments of small shelly fossils.The group has been compared with a number of animal phyla such as molluscs and brachiopods and the mystery surrounding the group has become a hot topic in recent paleontological research.Here,we concentrate on hyoliths with exquisite soft tissue preservation from Cambrian Series 2 Konservat-Lagerst?tten from South China,such as the Chengjiang and Guanshan biotas,taking advantage of the soft-part preservation to further explore hyolith anatomy.Combined with data from previous reports,the taxonomy,taphonomy and paleoecology of hyoliths will be explored and discussed in detail.In the present study,3 genera and 3 species are systematically revised and described from the Chengjiang biota,including Triplicatella opimus,Pedunculotheca diania,and Paramicrocornus ventricosus,(in addition to the problematic ‘Burithes yunnanensis’ the biological characteristics of which will need to be determined by further work).The species,‘Linevitus’ malongensis,from the Guanshan biota is also systematically investigated here.The hyoliths of the Chengjiang fauna(Cambrian Stage 3)are dominated by orthothecids,while only helens-bearing hyolithids are present in the Guanshan fauna(Cambrian Stage 4).Hyolith materials from these two South Chinese biotas reflect the evolutionary history of hyoliths and their morphological development in the early Cambrian,giving us an evolutionary model for how the group originated and diversified during the early stages of the Cambrian radiation.There are three points of new insight for the soft tissue preservation of hyolith:1.Triplicatella from the Chengjiang biota,is the first orthothecid which preserves a fanshaped feeding apparatus with a tuft-like arrangement of tentacles.Combined with the spiral-loop folded gut and morphological features of flat venter,Triplicatella is interpreted as a benthic deposit feeder,that could collect food directly from the substrate.The detailed comparisons of tentacles of hyoliths and members of lophophorates,combined with new evidence from shell microstructures reveal a high similarity between hyoliths and molluscs,indicating that hyoliths have a closer relationship with molluscs rather than with lophophorates.2.The typical hyolithid ‘Linevitus’ malongensis from the Guanshan biota is remarkably preserved with helens and internal soft tissues,including muscle scars and digestive tracts.The digestive tracts of ‘Linevitus’ malongensis show the typcial U-shape of hyolithids,and when compared with younger hyolithid specimens,the newly collected material indicates that the gut anatomy of hyolithids was evolutionarily conservative from the early Cambrian through to at least the Ordovician.3.The guts of hyoliths from the Guanshan biota in the new Shijiangjun section shows three types of preservational mode:(1)preservation through pyritization,(2)sediment-infilling of guts,and(3)bacterial biofilm pseudomorphs,resulting from endogenous bacterial decay.The material also yields some soft-bodied fossil concentrations dominated by hyoliths in both Chengjiang and Guanshan biotas,which were categorized previously as three types(elliptical aggregates;elongate,ribbon-like aggregates;concentric aggregates).In addition,the hyoliths from the Chengjiang biota are preferential hosts for other animal epibionts,such as brachiopods and molluscs,etc.These fossils reveal new information on the the role of hyoliths and benthic communities in the palaeozoic,and suggest that a complex food chain had already formed and testifies to an unprecedented expansion of ecological interactions in the early Cambrian.Two fresh research views on the evolution of hyolith shell as follows:1.A small hyolithid from the Chengjiang Biota was previously reported as Ambrolinevitus ventricosus.Here,this hyolithid is revised as Paramicrocornus ventricosus due to the high similarity of the opercula to Paramicrocornus from the Shuijingtuo Formation(Cambrian Series 2).A new family,the Paramicrocornuidae is proposed here for a group of hyoliths representing an intermediate stage in the evolution between Orthothecida and Hyolithida.This family shows a conch and operculum with a hyolithid morphology but lacks the characteristic helens that hyolithids generally possess.2.A phylogenetic analysis was first undertaken at a generic-level,that includes a range of Cambrian hyolithid and orthothecid taxa.The generated phylogenetic tree,together with the distribution of different hyoliths genera associated with strata of different ages,reveals a presumptive evolutionary model for hyoliths based on both the skeletal and soft anatomy.The character polarity of hyolith skeletons presents a transition from simple to complex,and indicates that hyoliths were derived from orthothecid ancestors with a simple-skeleton,with the later development of inner structures on the opercula,followed by the evolution of the typical hyolithid morphology with helens.The feeding habits are reflected from the different morphological functions,with orthothecids generally considered as deposit feeders,and the typical hyolithids,supported by helens,using the filter-feeding strategy.Thus,the evolution of the hyolith life mode shows a change from active deposit feeding to passive suspension/filter feeding.In the fossil record,the oldest hyoliths(orthothecids)occurred in the middle of the Fortunian Stage and radiated rapidly during the Cambrian explosion,until their ultimate demise at the end of the Permian.Analyses of data from the Paleobiology Database(PBDB)shows that the radiation and evolution of hyoliths can be recognized in three episodes:(1)the prelude episode of the Cambrian Explosion(Terreneuvian),i.e.,the diversification of“small shelly fossils”(SSFs);(2)the main episode of the Cambrian explosion(Series 2);(3)and the end episode of the Cambrian explosion(Miaolingian Series)through to the extinction in Permian.Hyoliths first appeared in the Terreneuvian and reached the highest diversity and disparity at the middle of Cambrian Series 2.After this peak in diversity,their diversity decreased dramatically during the later half of Stage 4,which may be closely related to the Botomian-Toyonian Extinction event.