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）”这一由达尔文首次提出的术语，被定义为近期物种多样化程度有限、且在漫长进化历程中形态停滞性极高的物种。然而，形态停滞性可能会导致物种多样化速率被低估。背甲目（Notostraca），即蝌蚪虾，是一类古老且全球分布的鳃足纲甲壳动物，因其丰富的化石记录可追溯至早泥盆世且形态高度保守，被视为“活化石”类群。近期的系统发育重建研究显示出显著的生物地理信号，表明其物种多样化由大陆漂移驱动，且存在广泛的隐存物种形成现象。但形态保守性使得将化石类群纳入系统发育框架的工作难度极大。本研究首次通过构建稳健的鳃足纲多位点系统发育树，并采用背甲目之外的可靠化石校准点开展贝叶斯分化时间估算，揭示了蝌蚪虾物种多样化的时间与演化速率。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”概念在隐存物种形成广泛存在的类群中的有效性提出了质疑。