Lamsdell James C
Department of Geology and Geography, West Virginia University, Morgantown, WV, United States of America.
PeerJ. 2020 Dec 4;8:e10431. doi: 10.7717/peerj.10431. eCollection 2020.
Xiphosurans are aquatic chelicerates with a fossil record extending into the Early Ordovician and known from a total of 88 described species, four of which are extant. Known for their apparent morphological conservatism, for which they have gained notoriety as supposed 'living fossils', recent analyses have demonstrated xiphosurans to have an ecologically diverse evolutionary history, with several groups moving into non-marine environments and developing morphologies markedly different from those of the modern species. The combination of their long evolutionary and complex ecological history along with their paradoxical patterns of morphological stasis in some clades and experimentation among others has resulted in Xiphosura being of particular interest for macroevolutionary study. Phylogenetic analyses have shown the current taxonomic framework for Xiphosura-set out in the in 1955-to be outdated and in need of revision, with several common genera such as Dunbar, 1923 and Størmer, 1952 acting as wastebasket taxa. Here, an expanded xiphosuran phylogeny is presented, comprising 58 xiphosuran species as part of a 158 taxon chelicerate matrix coded for 259 characters. Analysing the matrix under both Bayesian inference and parsimony optimisation criteria retrieves a concordant tree topology that forms the basis of a genus-level systematic revision of xiphosuran taxonomy. The genera Meek, 1867, König, 1820, , , and are demonstrated to be non-monophyletic and the previously synonymized genera Raymond, 1944 and Cockerell, 1905 are shown to be valid. In addition, nine new genera ( gen. nov. gen. nov. and gen. nov. in Belinurina; gen. nov. in Paleolimulidae; gen. nov. and gen. nov. in Austrolimulidae; and gen. nov., gen. nov., and gen. nov. in Limulidae) are erected to accommodate xiphosuran species not encompassed by existing genera. One new species, gen. et sp. nov., is also described. Three putative xiphosuran genera- Raymond, 1944, Chlupáč, 1963, and Chlupáč, 1963-are determined to be non-xiphosuran arthropods and as such are removed from Xiphosura. The priority of König, 1820 over Pictet, 1846 is also confirmed. This work is critical for facilitating the study of the xiphosuran fossil record and is the first step in resolving longstanding questions regarding the geographic distribution of the modern horseshoe crab species and whether they truly represent 'living fossils'. Understanding the long evolutionary history of Xiphosura is vital for interpreting how the modern species may respond to environmental change and in guiding conservation efforts.
剑尾目动物是水生螯肢动物，其化石记录可追溯到早奥陶世，目前已知有88个已描述物种，其中4种现存。剑尾目动物以其明显的形态保守性而闻名，它们因此被称为所谓的“活化石”而声名狼藉。最近的分析表明，剑尾目动物有着生态多样的进化史，有几个类群进入了非海洋环境，并进化出与现代物种明显不同的形态。它们漫长的进化历程和复杂的生态历史，再加上一些类群形态停滞而另一些类群形态多变的矛盾模式，使得剑尾目动物成为宏观进化研究的特别关注点。系统发育分析表明，1955年确立的剑尾目现行分类框架已经过时，需要修订，一些常见属，如1923年的邓巴属和1952年的斯托默属，都成了“废纸篓分类单元”。在此，我们展示了一个扩展的剑尾目系统发育树，包含58个剑尾目物种，作为一个由158个分类单元组成的螯肢动物矩阵的一部分，该矩阵编码了259个特征。在贝叶斯推断和简约优化标准下对该矩阵进行分析，得到了一个一致的树形拓扑结构，这构成了剑尾目分类学属级系统修订的基础。结果表明，1867年的米克属、1820年的柯尼格属、以及其他几个属并非单系类群，而先前被同义化的1944年的雷蒙德属和1905年的科克雷尔属是有效的。此外，还建立了9个新属（贝林鲎科中的新属、新属和新属；古鲎科中的新属；澳鲎科中的新属和新属；鲎科中的新属、新属和新属），以容纳现有属未涵盖的剑尾目物种。还描述了一个新物种，新属新种。三个疑似剑尾目属——1944年的雷蒙德属、1963年的赫卢帕奇属和1963年的赫卢帕奇属——被确定为非剑尾目节肢动物，因此从剑尾目中移除。同时也确认了1820年的柯尼格属优先于1846年的皮克泰属。这项工作对于促进剑尾目化石记录的研究至关重要，是解决关于现代鲎物种地理分布以及它们是否真的代表“活化石”等长期问题的第一步。了解剑尾目的漫长进化历史对于解释现代物种如何应对环境变化以及指导保护工作至关重要。
