Data S4 from A new ophiocistioid with soft-tissue preservation from the Silurian Herefordshire Lagerstätte, and the evolution of the holothurian body plan.

Reconstructing the evolutionary assembly of animal body plans is challenging when there are large morphological gaps between extant sister taxa, as in the case of echinozoans (echinoids and holothurians). However, the inclusion of extinct taxa can help bridge these gaps. Here we describe a new species of echinozoan, Sollasina cthulhu, from the Silurian Herefordshire Lagerstätte, UK. Sollasina cthulhu belongs to the ophiocistioids, an extinct group that shares characters with both echinoids and holothurians. Using physical–optical tomography and computer reconstruction, we visualize the internal anatomy of S. cthulhu in three dimensions, revealing inner soft tissues that we interpret as the ring canal, a key part of the water vascular system that was previously unknown in fossil echinozoans. Phylogenetic analyses strongly suggest that Sollasina and other ophiocistioids represent a paraphyletic group of stem holothurians, as previously hypothesized. This allows us to reconstruct the stepwise reduction of the skeleton during the assembly of the holothurian body plan, which may have been controlled by changes in the expression of biomineralization genes.

当现生姊妹类群间存在显著形态学间隙时，重建动物躯体构型的演化组装过程极具挑战——以游移支（echinozoans）为例，该类群包含现生的海胆纲（echinoids）与海参纲（holothurians），类群间便存在较大的形态学鸿沟。然而，纳入已灭绝类群可有效填补这些研究空白。本文记述了一件产自英国志留纪赫里福德郡特异埋藏化石群（Herefordshire Lagerstätte）的游移支新物种：梭拉西娜·克苏鲁（Sollasina cthulhu）。该物种隶属于蛇海胆类（ophiocistioids），这是一类已灭绝的类群，兼具海胆纲与海参纲的形态特征。研究团队借助物理光学层析成像（physical–optical tomography）与计算机重建技术，对该物种的内部解剖结构进行了三维可视化，揭示出其内部软组织被鉴定为环水管（ring canal）——这是水管系统（water vascular system）的关键组成部分，此前在化石游移支类群中从未被报道。系统发育分析（phylogenetic analyses）结果强烈支持梭拉西娜属（Sollasina）与其他蛇海胆类构成干群海参类（stem holothurians）的并系群（paraphyletic group），这与此前的学术假说一致。据此，我们得以重建海参躯体构型演化过程中骨骼的逐步退化步骤，该过程可能受生物矿化基因（biomineralization genes）的表达变化调控。