Du Kun-Sheng, Guo Jin, Losso Sarah R, Pates Stephen, Li Ming, Chen Ai-Lin
Research Center of Paleobiology, Yuxi Normal University, Yuxi, China.
Key Laboratory for Palaeobiology and MEC International Joint Laboratory for Palaeoenvironment, Institute of Palaeontology, Yunnan University, Kunming, China.
Elife. 2024 Oct 2;12:RP93113. doi: 10.7554/eLife.93113.
Euarthropods are an extremely diverse phylum in the modern, and have been since their origination in the early Palaeozoic. They grow through moulting the exoskeleton (ecdysis) facilitated by breaking along lines of weakness (sutures). Artiopodans, a group that includes trilobites and their non-biomineralizing relatives, dominated arthropod diversity in benthic communities during the Palaeozoic. Most trilobites - a hyperdiverse group of tens of thousands of species - moult by breaking the exoskeleton along cephalic sutures, a strategy that has contributed to their high diversity during the Palaeozoic. However, the recent description of similar sutures in early diverging non-trilobite artiopodans means that it is unclear whether these sutures evolved deep within Artiopoda, or convergently appeared multiple times within the group. Here, we describe new well-preserved material of , a putative early diverging artiopodan, including hitherto unknown details of its ventral anatomy and appendages revealed through CT scanning, highlighting additional possible homologous features between the ventral plates of this taxon and trilobite free cheeks. We used three coding strategies treating ventral plates as homologous to trilobite-free cheeks, to trilobite cephalic doublure, or independently derived. If ventral plates are considered homologous to free cheeks, is recovered sister to trilobites, however, dorsal ecdysial sutures are still recovered at many places within Artiopoda. If ventral plates are considered homologous to doublure or non-homologous, then is not recovered as sister to trilobites, and thus the ventral plates represent a distinct feature to trilobite doublure/free cheeks.
真节肢动物门在现代是一个极其多样化的门类，自其在古生代早期起源以来一直如此。它们通过沿着薄弱线（缝）断裂来促进外骨骼蜕皮（蜕壳）从而生长。三叶形纲动物，包括三叶虫及其非生物矿化的亲属，在古生代底栖生物群落中主导了节肢动物的多样性。大多数三叶虫——一个拥有数万物种的高度多样化群体——通过沿着头部缝线断裂外骨骼来蜕皮，这一策略促成了它们在古生代的高度多样性。然而，最近在早期分化的非三叶虫三叶形纲动物中发现了类似的缝线，这意味着尚不清楚这些缝线是在三叶形纲内部深处演化而来，还是在该类群中多次趋同出现。在这里，我们描述了一种推测为早期分化的三叶形纲动物的新的保存完好的材料，包括通过CT扫描揭示的其腹部解剖结构和附肢迄今未知的细节，突出了该分类单元腹侧板与三叶虫游离颊之间额外可能的同源特征。我们使用了三种编码策略，将腹侧板视为与三叶虫游离颊同源、与三叶虫头部内缘同源或独立演化而来。如果将腹侧板视为与游离颊同源，那么该动物被恢复为三叶虫的姐妹群，然而，在三叶形纲的许多地方仍然发现了背侧蜕皮缝线。如果将腹侧板视为与内缘同源或非同源，那么该动物就不会被恢复为三叶虫的姐妹群，因此腹侧板代表了与三叶虫内缘/游离颊不同的特征。
