Phylogenetic affinities and evolution of the Early Cambrian hexangulaconulariids

The abrupt appearance of abundant small shelly fossils (SSFs) near the base of the Cambrian System signals a key event in the evolutionary history of the Kingdom Metazoa. Among the probable cnidarian taxa, the SSF family Hexangulaconulariidae (Terreneuvian Series) is characterized by a fan-shaped, biradially symmetrical periderm having distinct apical and abapical portions, the latter of which bears multiple faces. Documented herein are three new hexangulaconulariid specimens from Cambrian Stage 2 on the Yangtze Platform (South China). The new specimens exhibit nine or 11 faces on each of the two sides of the abapical portion. Results of a Bayesian analysis of the phylogenetic relationships among hexangulaconulariids, carinachitids, conulariids, olivooids, and extant cnidarian genera indicate that hexangulaconulariids constitute a well-defined clade within a polytomic, stem-group Medusozoa that also includes carinachitids, conulariids and olivooids. The evolution of hexangulaconulariids featured multiple instances of increase in the total number of faces, modification of the morphology of the interfacial boundaries, and increase in the total size of the periderm. Increase in the number of faces appears to have been achieved in part by the transformation of a single ancestral face into three faces, through the development of a longitudinal furrow or ridge along each of the two adapically converging loci of inflection points (angular bends) of the prominent transverse ribs of the ancestral face.

寒武系底部附近大量小型壳体化石（small shelly fossils, SSFs）的突然出现，标志着动物界（Metazoa）演化历史中的关键事件。在可能的刺胞动物类群中，特伦纽统（Terreneuvian Series）的六棱锥管虫科（Hexangulaconulariidae）以扇形、两辐射对称的围鞘（periderm）为典型特征，其具有清晰的顶端与反顶端部分，后者发育多个面。本文记述了采自华南扬子地台（Yangtze Platform）寒武纪第二阶（Cambrian Stage 2）的3件六棱锥管虫科新标本。这些新标本的反顶端部分两侧各具有9或11个面。对六棱锥管虫科、脊锥管虫类、锥管虫类、橄榄形化石类以及现生刺胞动物属的系统发育关系（phylogenetic relationships）开展贝叶斯分析（Bayesian analysis），结果显示：六棱锥管虫科构成多歧分枝的水母型动物干群（stem-group Medusozoa）中的一个明确演化支（clade），该干群同时包含脊锥管虫类、锥管虫类与橄榄形化石类。六棱锥管虫科的演化过程存在多项特征变化：围鞘总面数多次增加、界面边界形态发生改造，以及围鞘整体尺寸增大。面数的增加似乎部分通过以下途径实现：沿祖先单个面的显著横肋（transverse ribs）的两个朝向顶端汇聚的拐点（inflection points，角弯）位置，分别发育纵向沟槽或脊，使单个祖先面转变为3个面。