Department of Geology and Biodiversity Institute, University of Kansas, Lawrence, Kansas, United States of America.
PLoS One. 2011;6(6):e21304. doi: 10.1371/journal.pone.0021304. Epub 2011 Jun 27.
Sphaerexochinae is a speciose and widely distributed group of cheirurid trilobites. Their temporal range extends from the earliest Ordovician through the Silurian, and they survived the end Ordovician mass extinction event (the second largest mass extinction in Earth history). Prior to this study, the individual evolutionary relationships within the group had yet to be determined utilizing rigorous phylogenetic methods. Understanding these evolutionary relationships is important for producing a stable classification of the group, and will be useful in elucidating the effects the end Ordovician mass extinction had on the evolutionary and biogeographic history of the group.
METHODOLOGY/PRINCIPAL FINDINGS: Cladistic parsimony analysis of cheirurid trilobites assigned to the subfamily Sphaerexochinae was conducted to evaluate phylogenetic patterns and produce a hypothesis of relationship for the group. This study utilized the program TNT, and the analysis included thirty-one taxa and thirty-nine characters. The results of this analysis were then used in a Lieberman-modified Brooks Parsimony Analysis to analyze biogeographic patterns during the Ordovician-Silurian.
CONCLUSIONS/SIGNIFICANCE: The genus Sphaerexochus was found to be monophyletic, consisting of two smaller clades (one composed entirely of Ordovician species and another composed of Silurian and Ordovician species). By contrast, the genus Kawina was found to be paraphyletic. It is a basal grade that also contains taxa formerly assigned to Cydonocephalus. Phylogenetic patterns suggest Sphaerexochinae is a relatively distinctive trilobite clade because it appears to have been largely unaffected by the end Ordovician mass extinction. Finally, the biogeographic analysis yields two major conclusions about Sphaerexochus biogeography: Bohemia and Avalonia were close enough during the Silurian to exchange taxa; and during the Ordovician there was dispersal between Eastern Laurentia and the Yangtze block (South China) and between Eastern Laurentia and Avalonia.
Sphaerexochinae 是 Cheirurid 三叶虫的一个多态且广泛分布的群体。它们的时间范围从最早的奥陶纪延伸到志留纪，并且在奥陶纪末期大灭绝事件（地球历史上第二大灭绝事件）中幸存下来。在这项研究之前，该群体内部的个体进化关系尚未通过严格的系统发育方法来确定。了解这些进化关系对于产生该群体的稳定分类非常重要，并且对于阐明奥陶纪末期大灭绝对该群体的进化和生物地理历史的影响也将很有用。
方法/主要发现：对归入 Sphaerexochinae 亚科的 Cheirurid 三叶虫进行了分支系统学简约分析，以评估进化模式并提出该群体的关系假说。本研究使用了 TNT 程序，分析包括 31 个分类单元和 39 个特征。然后，将该分析的结果用于 Lieberman 修正的 Brooks 简约分析，以分析奥陶纪-志留纪的生物地理模式。
结论/意义：发现 Sphaerexochus 属是单系的，由两个较小的分支组成（一个完全由奥陶纪物种组成，另一个由志留纪和奥陶纪物种组成）。相比之下，Kawina 属被发现是并系的。它是一个基础等级，还包含以前归入 Cydonocephalus 的分类单元。系统发育模式表明 Sphaerexochinae 是一个相对独特的三叶虫类群，因为它似乎在很大程度上未受到奥陶纪末期大灭绝的影响。最后，生物地理分析得出了关于 Sphaerexochus 生物地理的两个主要结论：志留纪时期，波希米亚和阿瓦隆尼亚足够接近，可以交换分类单元；奥陶纪时期，东劳伦西亚和扬子地块（华南）之间以及东劳伦西亚和阿瓦隆尼亚之间存在扩散。
