The Evolution And Radiation Of Hexangulaconulariids From Cambrian Terreneuvian,South China

The abrupt appearance of small shelly fossils(SSFs)in the early Cambrian is a significant evolutionary event of metazoan.Small shelly fossils witness the multi-episodes of the Cambrian Explosion,and provide a key window to study the metazoan evolution on the eve of the second episode of the Cambrian Explosion.Among diverse SSFs,hexangulaconulariids are a group of extinct cnidarians with a biradially-symmetrical body plan,and are a major component of early Cambrian marine benthic communities.Due to material limitation,the previous studies of hexangulaconulariids most focused on the morphological descriptions and systematic palaeontology.Other aspects of hexangulaconulariids,such as ontogeny,evolution and radiation pattern,were quite scarcely analyzed in literature.Herein,based on the abundant materials of hexangulaconulariids collected from the Terreneuvian Yanjiahe Formation in Three Gorges,and from the Fortunian in southern Shaanxi,northern Sichuan,western Sichuan and eastern Yunnan,a comprehensive study of hexangulaconulariids has been conducted in many aspects,e.g.,systematic palaeontology,ontogeny,phylogeny,etc.Stratigraphic successions of hexangulaconulariids and their spatiotemporal distribution patterns are also analyzed on the basis of these materials.The research results are as follows:Hexangulaconulariids from the Terreneuvian in South China are systematically described,including 1 family,6 genera and 8 species.The family Hexangulaconulariidae He,1987 currently contains,six species and two unnamed new genera and species(A and B):Arthrochites emeishanensis Chen,1982,Hexangulaconularia formosa He in Xing et al,1983 emend.,Decimoconularia anisfacialis Song et al.,2023,D.isofacialis Guo et al.,2021,Septuconularia yanjiaheensis Guo et al.,2020,S.crassiformis Song et al.,2022.According to the priority principle of the International Code of Zoological Nomenclature(ICZN),the previous genus Hexaconularia He and Yang,1986 is taken as a junior synonym of Hexangulaconularia He in Xing et al.,1983 and the type specie Hexaconularia sichuanensis He and Yang,1986 is considered as a junior synonym of Hexangulaconularia formosa He in Xing et al.,1983.In addition,two new forms respectively exhibiting 18 and 22 faces are taken as new genera and species but unnamed here(A and B)due to the limited number of specimens.The ontogeny of hexangulaconulariids is of a direct developmental pattern,and includes three different developmental stages: embryonic,juvenile and adult stage.The growth pattern of periderm is inferred to be a continuous growing along oral-aboral axis,and the variable space between transverse ribs reflects the rate of growth.In addition,the periderm of hexangulaconulariids is phosphatized,and exhibits a mono-layered tough wall,which is supported by the skeleton of longitudinal ridges or furrows.The life habit of hexangulaconulariids is sessile benthic with the apex of apical region attached to hard substrates by secreting mucus.Moreover,the laterally compressed biradially-symmetrical body plan of hexangulaconulariids may be more suitable for the benthic sessile life habit in a high-energy hydrodynamic environment.Finally,the narrowly lenticular shape of apertural margin may effectively reduce the entry of sedimentary particles and broken shells into the inner cavity,which is conducive to the filter feeding habit of hexangulaconulariids in the shallow sea shelf.Hexangulaconulariids are mainly reported in northern and western Yangtze platform,the stratigraphic distributions of that are restricted from Cambrian Fortunian to Stage 2.The morphological diversity change of hexangulaconulariids reveals its evolutionary process of hexangulaconulariids from Fortunian to Stage 2.This process is achieved by the collinear ‘sharp bend’ structure formed in the central face.Thus,the evolution model of hexangulaconulariids is that the number of faces on one side of periderm shows a continuous accretion of the odd type(1→3→5→7→9→11)and the longitudinal structure dividing faces shows a continuous transition(longitudinal furrow→ collinear longitudinal furrow and ridge→ longitudinal ridge)in the Cambrian Terreneuvian.The phylogenetic position of hexangulaconulariids is indicated by reconstructing phylogenetic tree based on traditional criteria of homology and phylogenetic analysis.The results show that hexangulaconulariids can be assigned to a stem group Medusozoa,and the more exact phylogenetic position need to be determined by the discovery of more specimens with information about soft tissue structure and anatomy.