Abstract
Dating the Opiliones tree of life has become an important enterprise for this group of arthropods, due to their ancient origins and important biogeographic implications. To incorporate both methodological innovations in molecular dating as well as new systematic discoveries of harvestman diversity, we conducted total evidence dating on a data set uniting morphological and/or molecular sequence data for 47 Opiliones species, including all four well-known Palaeozoic fossils, to test the placement of both fossils and newly discovered lineages in a single analysis. Furthermore, we investigated node dating with a phylogenomic data set of 24,202 amino acid sites for 14 species of Opiliones, sampling all extant suborders. In this way, we approached molecular dating of basal harvestman phylogeny using different data sets and approaches to assess congruence of divergence time estimates. In spite of the markedly different composition of data sets, our results show congruence across all analyses for age estimates of basal nodes that are well constrained with respect to fossil calibrations (e.g., Opiliones, Palpatores). By contrast, derived nodes that lack fossil calibrations (e.g., the suborders Cyphophthalmi, and Laniatores) have large uncertainty intervals in diversification times, particularly in the total evidence dating analysis, reflecting the dearth of calibration points and undersampling of derived lineages. Total evidence dating consistently produced older median ages than node dating for ingroup nodes, due to the nested placement of multiple Palaeozoic fossils. Our analyses support basal diversification of Opiliones in the Ordovician-Devonian period, corroborating the inferred ancient origins of this arthropod order, and underscore the importance of diversity discovery—both paleontological and neontological—in evolutionary inference.
Highlights
Dating molecular phylogenies has the power to provide an evolutionary framework for a group in question, beyond inference of relationships alone (Benton, 1995; Wang et al, 1999)
In accordance with the systematic treatment of Groh and Giribet (2014), Hesperopilio was recovered as nested within Phalangioidea (PP = 1.00); Caddidae sensu stricto was composed of only the genus Caddo, the sister group to the remaining Eupnoi; and Acropsopilionidae was recovered as sister group to the remaining Dyspnoi
Macrogyion cronus is very likely a member of Phalangioidea, but is difficult to place using the morphological matrix because the accessory tibial spiracles—a diagnostic synapomorphy of the superfamily—cannot be observed in the 3D reconstruction of this fossil’s appendages (Garwood et al, 2011)
Summary
Dating molecular phylogenies has the power to provide an evolutionary framework for a group in question, beyond inference of relationships alone (Benton, 1995; Wang et al, 1999). The fossil record has been used in a diversity of manners, either to constraint nodes (“node dating”), or using fossils as terminals in combined analyses of morphology and molecules (“total evidence dating”; Murienne et al, 2010; Pyron, 2011; O’Leary et al, 2013) The effects of both practices are just beginning to be evaluated (Wood et al, 2013), but is clear that special attention should be paid to the actual fossils used for calibration, as precise dating and accurate phylogenetic placement of the fossil material has important effects on the final results of the analyses, demanding standards that had been largely ignored until recently (Parham et al, 2012). Recent studies have evaluated a range of dating techniques and calibrations to better test biogeographic hypotheses (Mao et al, 2012; Murienne et al, 2014), a practice that should become more widespread in future investigations of evolutionary history
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