A revised dated phylogeny of the arachnid order Opiliones

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.

Cross-bracing uncalibrated nodes in molecular dating improves congruence of fossil and molecular age estimates

“…In that study, the age of spider diversification was estimated at 325-339 Ma in two Bayesian analyses (95% HPD interval: 305-387 Ma, across both analyses), a result very consistent with the one obtained in the present study. The congruence of results from these very disparate data sets [31,56] reinforces the tenet that proper algorithmic treatment of fossil taxa is far more important for molecular dating than quantity of sequence data [56].…”

“…To our knowledge, the only molecular dating available for the basal split of scorpions (between buthids and allies, and the remaining scorpions) was conducted by Rehm et al [31] and Sharma and Giribet [56]. In the former study, as only a single buthid sequence (Androctonus australis Hc6) was available, this split was not as well represented as the spider divergences in that dataset.…”

“…By contrast, a phylogenomic analysis by Sharma and Giribet [56] focusing on the internal dating of Opiliones (based on 3,644 loci) also included among the outgroups exemplars of all three major spider lineages, Amblypygi, and Uropygi, and employed the age of the earliest Mesothelae as a minimum age constraint for spider divergence (in addition to separate calibrations within Opiliones). In that study, the age of spider diversification was estimated at 325-339 Ma in two Bayesian analyses (95% HPD interval: 305-387 Ma, across both analyses), a result very consistent with the one obtained in the present study.…”

“…Another two recent studies of arachnid relationships at the transcriptomic scale dated the phylogeny of Araneae and included an exemplar of Mesothelae [55,56]. However, in the first case, Bond et al [55] dated the tree with 128 loci in the absence of any outgroups (a pseudoscorpion, a tick, and a water flea used in phylogenomic analyses were culled from the dating analysis), and did not include closely related taxa such as amblypygids or uropygids.…”

Phylogenomic resolution of scorpions reveals multilevel discordance with morphological phylogenetic signal

“…direct sperm transfer; median ocelli; paired tarsal claws), and spiders (e.g. unsegmented opisthosoma; venom glands; labidognathous chelicerae), and these evolutionary trends have been robustly validated by phylogenomic data [14,54,[59][60][61][62][63][64]. In the case of scorpions, barring the clear separation of buthids from non-buthid lineages, there has been little agreement as to how scorpion families are related.…”