Hyolith fauna from the Xinji Formation (Cambrian Series 2) in North China

Hyoliths are one of the prominent components of the Cambrian small shelly fossil assemblage of the North China Plate and witnessed the explosive radiation of animals in North China. Here, fourteen species and one undetermined species assigned to nine genera of the Hyolitha are systematically described. These fossils are collected from the Xinji Formation (Series 2, Cambrian) along the south-southwest margin of the plate. This hyolith fauna yields nine orthothecid species in six genera: Conotheca australiensis, Cupitheca holocyclata, Cupitheca costellata, Cupitheca? decollata, Tegminites hymenodes, Triplicatella disdoma, Triplicatella xinjia, Protomicrocornus triplicensis, and Longxiantheca mira; five hyolithid species in three genera and one undetermined species: Microcornus eximius, Microcornus petilus, Parkula cf. esmeraldina, Parkula bounites, Parakorilithes mammillatus, and Parakorilithes sp. Based on abundant well-preserved articulated specimens of this assemblage, we provide a complete reconstruction of the hyolith skeletons (conch-operculum). In addition, more than half of the studied hyolith species in North China possess transverse septa within their conchs. Of these, septa in Conotheca and Longxiantheca can effectively strengthen and further seal off the apical ends of the conchs, and are used for decollation during growth of the conchs in Cupitheca and Microcornus. Moreover, conchs of Parakorilithes and Protomicrocornus develop single and multiple septa respectively, which can largely avoid penetration of soft tissues into the conical shells. Based on these observations, we propose that the presence of teeth-like structures of operculum, septa and unique decollating growth mode of the conch can significantly enhance the sealing of the conch by associated operculum, and improve the adaptability of calcareous exoskeletons to marine substrate environments. Meanwhile, the tentacle-bearing feature of soft tissues functions as a suspension-scavenging-detritus feeding apparatus and is adapted to a wide spectrum of niches. These survival strategies and evolutionary adaptability may be one of the important reasons for the rapid diversification and thriving of the hyoliths in the Cambrian ocean.

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