Lustri Lorenzo, Antcliffe Jonathan B, Gueriau Pierre, Daley Allison C
Institute of Earth Sciences, University of Lausanne, Géopolis, Lausanne CH-1015, Switzerland.
Université Paris-Saclay, CNRS, ministère de la Culture, UVSQ, MNHN, Institut photonique d'analyse non-destructive européen des matériaux anciens, Saint-Aubin 91192, France.
R Soc Open Sci. 2024 Oct 30;11(10):240499. doi: 10.1098/rsos.240499. eCollection 2024 Oct.
The rapid early diversification of arthropods has made understanding internal relationships within the group fiendish. Particularly unresolved is the origin of Euchelicerata, a clade consisting of the Prosomapoda (comprising the extant Xiphosura and Arachnida and the extinct Chasmataspidida, Eurypterida and synziphosurines) and the extinct Offacolidae. Here we describe new material of the Silurian 'synziphosurine' Clarke, 1919 that reveals previously unknown features of its ventral anatomy: a pair of elongated chelicerae in the prosoma, followed posteriorly by five pairs of biramous appendages, a first pre-abdomen somite bearing a pair of paddle-like uniramous appendages (exopods) and a ventral pretelsonic process. Phylogenetic analyses retrieve as a member of Offacolidae closely related to from the Early Ordovician Fezouata Biota. An anatomical comparison of the pretelsonic process of , also present in , with the posterior trunk morphologies of other Offacolidae, Habeliida and Vicissicaudata, suggests a possible homologous appendicular origin. This proposed apomorphic character supports a monophyletic Arachnomorpha, formed of Vicissicaudata, Habeliida and Euchelicerata. The establishment of this new homology could help to clarify the highly enigmatic phylogeny at the base of the euchelicerates as well as the sequence of character acquisition during their early evolution.
节肢动物在早期的快速多样化使得理解该类群内部的亲缘关系变得极为困难。尤螯肢亚门（Euchelicerata）的起源问题尤其难以解决，该演化支包括前体足纲（Prosomapoda，由现存的剑尾目（Xiphosura）和蛛形纲（Arachnida）以及已灭绝的裂口蛛目（Chasmataspidida）、广翅鲎目（Eurypterida）和近蛛形类（synziphosurines）组成）和已灭绝的奥法科蛛科（Offacolidae）。在此，我们描述了1919年命名的志留纪“近蛛形类”的新材料，该材料揭示了其腹侧解剖结构中一些前所未知的特征：前体中有一对细长的螯肢，其后依次是五对双分支附肢，第一腹前节有一对桨状单分支附肢（外肢）以及一个腹侧前尾突。系统发育分析将其归入与早奥陶世费祖阿塔生物群中的[具体物种]密切相关的奥法科蛛科。对同样存在于[另一物种]中的前尾突与其他奥法科蛛科、哈贝利目（Habeliida）和异尾类（Vicissicaudata）的躯干后部形态进行解剖学比较后发现，其可能起源于同源附肢。这一假定的衍征性状支持了由异尾类、哈贝利目和尤螯肢亚门组成的单系蛛形形态类（Arachnomorpha）。这一新同源性的确立有助于厘清尤螯肢亚门基部高度神秘的系统发育关系以及它们早期演化过程中特征获得的顺序。
