Abstract
‘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, is 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.
Highlights
There has been much debate about the tempo and mode of the diversification of life (Eldredge & Gould, 1972; Reznick & Ricklefs, 2009; Rhodes, 1983)
One extreme and often controversial pattern of diversification is found in ‘living fossils’, a concept introduced by Charles Darwin in On the Origin of Species when dealing with the perplexing nature of the platypus and lungfish, relicts of once diverse groups (Darwin, 1859)
Morphological stasis can obscure the patterns of species diversification, and recent time-calibrated phylogenetic analyses of some ‘living fossils’ have revealed that extant species are only recently diverged (Kano et al, 2012; Nagalingum et al, 2011)
Summary
There has been much debate about the tempo and mode of the diversification of life (Eldredge & Gould, 1972; Reznick & Ricklefs, 2009; Rhodes, 1983). The nomenclature and systematic position of some ancient extinct Notostraca lineages, is, problematic (Hegna & Dong, 2010) This is partly because tadpole shrimps have maintained an extremely conserved, yet complex, bauplan with extant species indistinguishable from fossils of Triops from the Triassic (Gall & Grauvogel-Stamm, 2005; Gore, 1986; Trusheim, 1938) and of Lepidurus in the Jurassic (Barnard, 1929; Haughton, 1924). This striking morphological conservatism has led them to be referred to as ‘living fossils’ (Fryer, 1988; King & Hanner, 1998; Mantovani et al, 2008). Our analysis uses all available Notostraca sequence data for seven genes, and Bayesian relaxed clock dating techniques, with multiple branchiopod fossil calibrations, to estimate divergence times
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