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

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

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