Data from: Multiple global radiations in tadpole shrimps challenge the concept of ‘living fossils’

‘Living fossils’, a phrase first coined by Darwin, are defined as species with limited recent diversification and high morphological stasis over long periods of evolutionary time. Morphological stasis, however, can potentially lead to diversification rates being underestimated. Notostraca, or tadpole shrimps, are an ancient, globally distributed order of branchiopod crustaceans regarded as ‘living fossils’ because their rich fossil record dates back to the early Devonian and their morphology is highly conserved. Recent phylogenetic reconstructions have shown a strong biogeographic signal, suggesting diversification due to continental breakup, and widespread cryptic speciation. However, morphological conservatism makes it difficult to place fossil taxa in a phylogenetic context. Here we reveal for the first time the timing and tempo of tadpole shrimp diversification by inferring a robust multilocus phylogeny of Branchiopoda and applying Bayesian divergence dating techniques using reliable fossil calibrations external to Notostraca. Our results suggest at least two bouts of global radiation in Notostraca, one of them recent, so questioning the validity of the ‘living fossils’ concept in groups where cryptic speciation is widespread.

“活化石（Living fossils）一词最早由达尔文提出，被定义为近期分化程度有限、且在漫长演化历程中形态停滞（morphological stasis）的物种类群。然而，形态停滞可能会导致物种分化速率被低估。背甲目（Notostraca）俗称蝌蚪虾，是一类古老且全球分布的鳃足纲（Branchiopoda）甲壳动物，被视为“活化石”：其丰富的化石记录可追溯至早泥盆世，且形态高度保守。近期的系统发育重建研究显示出强烈的生物地理信号，表明其物种分化与大陆裂解有关，且普遍存在隐存物种形成（cryptic speciation）。不过，形态保守性使得将化石类群纳入系统发育框架变得极为困难。本研究首次通过构建稳健的鳃足纲多位点系统发育，并利用背甲目以外的可靠化石校准点开展贝叶斯分化定年分析，揭示了蝌蚪虾物种分化的时间节点与分化速率。研究结果表明，背甲目至少经历了两次全球性辐射演化事件，其中一次发生在近期，这对“活化石”概念在普遍存在隐存物种形成的类群中的适用性提出了质疑。