Department of Zoology, University of Cambridge, Cambridge, UK.
Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China.
Nat Commun. 2022 Nov 15;13(1):6969. doi: 10.1038/s41467-022-34204-w.
A crucial step in the evolution of Euarthropoda (chelicerates, myriapods, pancrustaceans) was the transition between fossil groups that possessed frontal appendages innervated by the first segment of the brain (protocerebrum), and living groups with a protocerebral labrum and paired appendages innervated by the second brain segment (deutocerebrum). Appendage homologies between the groups are controversial. Here we describe two specimens of opabiniid-like euarthropods, each bearing an anterior proboscis (a fused protocerebral appendage), from the Middle Ordovician Castle Bank Biota, Wales, UK. Phylogenetic analyses support a paraphyletic grade of stem-group euarthropods with fused protocerebral appendages and a posterior-facing mouth, as in the iconic Cambrian panarthropod Opabinia. These results suggest that the labrum may have reduced from an already-fused proboscis, rather than a pair of arthropodized appendages. If some shared features between the Castle Bank specimens and radiodonts are considered convergent rather than homologous, phylogenetic analyses retrieve them as opabiniids, substantially extending the geographic and temporal range of Opabiniidae.
在真节肢动物（螯肢动物、多足动物、甲壳动物）的进化过程中，一个关键步骤是具有由脑第一部分（前脑）神经支配的额附肢的化石群与具有前脑背板和由第二脑部分（后脑）神经支配的成对附肢的现存群之间的转变。这些群之间的附肢同源性存在争议。本文描述了来自英国威尔士中奥陶统城堡银行生物群的两种类似 opabiniid 的真节肢动物标本，每个标本都有一个前吻（融合的前脑附肢）。系统发育分析支持具有融合的前脑附肢和向后开口的基干真节肢动物的并系等级，类似于标志性的寒武纪泛节肢动物 Opabinia。这些结果表明，背板可能是从前已融合的吻退化而来的，而不是一对节肢动物化的附肢。如果将 Castle Bank 标本和辐射栉水母之间的一些共同特征视为趋同而不是同源，系统发育分析将它们检索为 opabiniid，极大地扩展了 Opabiniidae 的地理和时间范围。
