Data S5 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, <i>Sollasina cthulhu</i>, from the Silurian Herefordshire Lagerstätte, UK. <i>Sollasina cthulhu</i> 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 <i>S. cthulhu</i> 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 <i>Sollasina</i> 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.

重建动物躯体构型的演化组装过程颇具挑战，尤其当现生姊妹类群间存在显著形态学鸿沟时，棘海胆亚门（echinozoans，涵盖海胆类与海参类）即为典型代表。然而，纳入灭绝类群可有效填补这类演化缺口。本文记述了采自英国志留纪赫里福郡特异埋藏化石库（Herefordshire Lagerstätte）的棘海胆亚门新物种**克苏鲁索拉辛海胆（Sollasina cthulhu）**。该新种隶属于蛇皮海胆类（ophiocistioids），这一已灭绝类群兼具海胆类与海参类的形态特征。本研究借助物理光学层析成像与计算机三维重建技术，对**克苏鲁索拉辛海胆（S. cthulhu）**的内部解剖结构开展三维可视化分析，揭示出其内部软组织，经鉴定为环管——步管系统（water vascular system）的关键组成部分，此类结构此前在棘海胆亚门的化石类群中尚未被报道过。系统发育分析结果强烈支持：索拉辛海胆属（Sollasina）与其他蛇皮海胆类构成海参类干群的并系群，这与此前的学术假说相符。这一发现使得我们得以重建海参类躯体构型演化过程中骨骼的逐步退化历程，该过程或受生物矿化基因表达调控的变化所驱动。