Deep origin of articulation strategies in panarthropods: evidence from a new luolishaniid lobopodian (Panarthropoda) from the Tulip Beds, Burgess Shale

The evolution of articulated sclerites via soft membranes, termed arthrodization, is arguably one of the most critical innovations in animals. Defining the megaphylum Arthropoda, the arthrodization of appendages, or arthropodization, likely predated that of the body, the combination of both being diagnostic of true arthropods (Euarthropoda) – all of these innovations occurring during the Cambrian explosion. Here, thanks to dozens of exceptionally preserved fossils from the Cambrian Wuliuan Stage Burgess Shale (Tulip Beds locality on Mount Stephen, British Columbia, Canada), we show that a distinct but comparable system of imbricated sclerotic elements evolved in the paraphyletic sister group of arthropods, the lobopodians. <i>Entothyreos synnaustrus</i> gen. et sp. nov. has characteristic body plan features of the Collinsovermidae (order Luolishaniida), including anterior limbs for suspension-feeding and stout anchoring posterior limbs. Uniquely, however, <i>E. synnaustrus</i> also displays segmental sclerotic sheets along the trunk, covered in a thin layer of integument, as well as overlapping sclerotized annuli on posterior-most limbs. While the latter elements likely served a protective function, the dorsolateral trunk sheets, which also carry spines, may have facilitated body erection and suspension-feeding. Other luolishaniids possess separate ring-like structures connecting the base of metameric spines which are covered by the apical layer of the lobopodian integument. <i>E. synnaustrus</i> and related taxa illustrate, therefore, an arguably parallel evolution of arthropod-like morphoanatomical features early during the rise of panarthropods. This finding broadens our perspectives on the uniqueness of major synapomorphies and the importance of including canalization in macroevolutionary narratives. http://zoobank.org/urn:lsid:zoobank.org.pub:D4A01784-E587-481A-AB04-A812B4AAE422

通过软组织膜连接的关节骨片演化（即关节化（arthrodization）），可说是动物界最关键的演化创新之一。这一特征界定了节肢动物总门（Megaphylum Arthropoda），而附肢的关节化（即附肢关节化（arthropodization））很可能早于躯干的关节化，二者的结合是真节肢动物亚门（Euarthropoda）——即真正的节肢动物——的鉴别特征，所有这些创新都发生在寒武纪大爆发（Cambrian explosion）期间。本次研究得益于加拿大不列颠哥伦比亚省斯蒂芬山郁金香床化石点产出的数十块保存极为精美的寒武纪伍里乌统阶布尔吉斯页岩（Burgess Shale）化石，我们证实，在节肢动物的并系姐妹群叶足动物（lobopodians）中，演化出了一套独特但结构类似的覆瓦状硬化骨片系统。新属新种*Entothyreos synnaustrus*具备科林森虫科（Collinsovermidae，罗留山虫目（Luolishaniida））的典型躯体构型特征，包括用于滤食的前附肢以及粗壮的锚定型后附肢。然而独特的是，*E. synnaustrus*还在躯干上分布有分节的硬化骨片层，其表面覆盖一层薄薄的表皮（integument），同时在最末端的附肢上带有相互重叠的硬化环纹（sclerotized annuli）。尽管后者大概率起到保护作用，但同样带有棘刺的躯干背外侧骨片层则可能辅助躯体直立以及滤食行为。其他罗留山虫类物种拥有独立的环状结构，连接着分节棘刺的基部，该结构被叶足动物表皮的顶端层所覆盖。因此，*E. synnaustrus*及其近缘类群展现了泛节肢动物（panarthropods）起源早期，类节肢动物形态解剖特征的平行演化，这一发现拓宽了我们对主要共源性状（synapomorphies）独特性的认知，也凸显了将渠化演化（canalization）纳入大演化叙事的重要性。http://zoobank.org/urn:lsid:zoobank.org.pub:D4A01784-E587-481A-AB04-A812B4AAE422