The Taphonomic Characteristics Of Soft-shelled Snail Fossils From The Cambrian Kaili Formation In Jianhe, Guizhou, And The Symbiotic Relationship With The Proto-crinoid

Hyoliths is a group of extincted lophotrochozoans with a life span ranging from the early Cambrian to the late Permian.With the earliest biomineralized shell widely distributed in Paleozoic strata,hyoliths is a key member of Cambrian ecology and has great potential in exploring Cambrian skeletonization.Here we present massive hyolith fossils in good condition collected from Kaili Formation,contributing to understanding hyoliths taxonomically and taphonomically,and discussing their symbiotic relationship with eocrinoids.Both Hyolithida and Orthothecida were produced in Kaili Formation,including 3genera and 3 species in total,namely Triplicatella opimus(Yu 1974),Paramicrocornus cf.ventricosus(Qian,1978)and Haplophrentis carinatus(Matthew,1899).The opercula,helens and conchs all were collected form Kaili Formation.Three preservation models for the opercula:(1)separately preserved opercula,(2)opercula completely articulated with conchs,(3)opercula partially articulated with conchs.The isolatedly preserved opercula are further classified as body fossil,internal mold and external mold,in which opercula body fossil was preserved with ornaments(fine growth lines)and corresponding pits of clavicle was observed on the internal mold.A few disarticulated helens were recognized.Three preservation models for the conchs:(1)internal mold of conchs,(2)external mold of conchs,(3)conchs with wall.Most of hyolith conchs were internal mold.In addition,a considerable number of impression-like hyolith fossils were also obtained in Kaili Formation,among which trident-like structures were observed within the impression,a typical taphonomic feature for the hyoliths in Kaili Formation.Kaili Formation yielded many other groups of fossils that were preserved with hyoliths,typically trilobites,brachiopods,eocinoids,macroalgae,Wiwaxia sclerites,and Scenella.Most of these groups scattered randomly around hyoliths.Additionally,quite a lot of aggregated hyoliths,with some in coprolites,were found,and among them the broken aggregated hyolith conchs in line suggest water transportation while the articulated hyoliths in coprolites represent a feeding strategy(behaviours)of Cambrian hyoliths.A certain amount of specimens display hyolithids attached by eocrinoids on both operculum and conch venter.The eocrinoid holdfasts left on operculum confirmed the symbiotic relationship between hyolithids and eocrinoids.It was found that the eocrinoids were immature(in the early stage of ontogeny)by referring to the ontogenetic features of stalked eocrinoids,shedding indirect light on the symbiotic time span prior to burial.Additionally,the orientation of the eocinoid buried in Konservat-Lagerst?tte implies the direction of mud flow during the incident-induced bury,aiding to understanding and possibly restoring the taphonomic process.Finally,based on many reliable specimens displaying eocrinoids attaching on different hyolithid conchs,three preservation models were established.It suggested that taphonomic factors led to the preservation artefact that epibionts preferred to attaching on the conch dorsal of hyolith,which was generally considered as a mutualism in previous works.