华北寒武系第二统辛集组软舌螺动物化石群

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

软舌螺动物是华北板块寒武系小壳化石基本组成之一,也是寒武纪生命大爆发在华北地区的重要见证者。本文系统描述了华北板块南缘-西南缘(河南-陕西地区)寒武系第二统辛集组软舌螺化石9属14种及1未定种。包括直管螺类6属9种:Conotheca australiensis、Cupitheca holocyclata、Cupitheca costellata、Cupitheca?decollata、Tegminites hymenodes、Triplicatella disdoma、Triplicatella xinjia、Protomicrocornus triplicensis和Longxiantheca mira;软舌螺类3属5种及1未定种:Microcornus eximius、Microcornus petilus、Parkula cf.esmeraldina、Parkula bounites、Parakorilithes mammillatus和Parakorilithes sp.。基于大量保存的精美聚合标本,本文准确重建了锥壳和口盖在软舌螺不同属种中的配置。此外,新材料显示华北辛集组软舌螺超过一半的属种钙质锥壳内侧发育隔板。其中,Conotheca和Longxiantheca隔板能有效加固和进一步封闭锥壳壳顶;Cupitheca和Microcornus沿隔板具断壳现象;Parakorilithes和Protomicrocornus分别发育特殊的单隔板和多隔板,能够很大程度上避免软体组织深入锥壳内部。笔者认为软舌螺通过口盖具齿、隔板加固、锥壳断壳等方式能有效增强锥壳-口盖闭合性和钙质外骨骼对海底环境的适应性,以及通过具触手营养器官进行滤食-腐食-掘食等不同方式生活。这些生存策略和适应性演化特征可能是软舌螺动物快速辐射并繁盛于寒武纪海洋的重要原因。

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.