A re‐interpretation of the ambulacral system of Eumorphocystis (Blastozoa, Echinodermata) and its bearing on the evolution of early crinoids

Recent debates over the evolutionary relationships of early echinoderms have relied heavily on morphological evidence from the feeding ambulacral system. Eumorphocystis, a Late Ordovician diploporitan, has been a focus in these debates because it bears ambulacral features that show strong morphological similarity to early crinoid arms. Undescribed and well‐preserved specimens of Eumorphocystis from the Bromide Formation (Oklahoma, USA) provide new data illustrating that composite arms supported by a radial plate that bear a triserial arrangement of axial and extraxial components encasing a coelomic extension can also be found in blastozoans. Previous reports have considered these arm structures to be restricted to crinoids; these combined features have not been previously observed in blastozoan echinoderms. Phylogenetic analyses suggest that Eumorphocystis and crinoids are sister taxa and that shared derived features of these taxa are homologous. The evidence from the arms of Eumorphocy...

近年来关于早期棘皮动物（echinoderms）演化亲缘关系的争论，高度依赖摄食步带系统的形态学证据。真囊球虫（Eumorphocystis）作为一种晚奥陶世双孔类棘皮动物，始终是此类争论的焦点，因其所具有的步带结构与早期海百合腕在形态学上具有高度相似性。采自美国俄克拉荷马州溴化物组（Bromide Formation）、尚未被正式描述且保存完好的真囊球虫标本，提供了新的研究数据：由辐板支撑的复合腕，其轴部与轴外组分呈三列排列并包裹体腔延伸结构，这一特征也存在于芽茎类棘皮动物（blastozoans）中。此前的研究均认为这类腕结构仅局限于海百合类群，此类组合特征此前从未在芽茎类棘皮动物中被观测到。系统发育分析结果显示，真囊球虫与海百合为姊妹群，二者的共源衍生特征具有同源性。来自真囊球虫腕部的证据……