Dating the origin of the major lineages of Branchiopoda

Despite the well-established phylogeny and good fossil record of branchiopods, a consistent macro-evolutionary timescale for the group remains elusive. This study focuses on the early branchiopod divergence dates where fossil record is extremely fragmentary or missing. On the basis of a large genomic dataset and carefully evaluated fossil calibration points, we assess the quality of the branchiopod fossil record by calibrating the tree against well-established first occurrences, providing paleontological estimates of divergence times and completeness of their fossil record. The maximum age constraints were set using a quantitative approach of Marshall (2008). We tested the alternative placements of Yicaris and Wujicaris in the referred arthropod tree via the likelihood checkpoints method. Divergence dates were calculated using Bayesian relaxed molecular clock and penalized likelihood methods. Our results show that the stem group of Branchiopoda is rooted in the late Neoproterozoic (563 ± 7 Ma); the crown-Branchiopoda diverged during middle Cambrian to Early Ordovician (478–512 Ma), likely representing the origin of the freshwater biota; the Phyllopoda clade diverged during Ordovician (448–480 Ma) and Diplostraca during Late Ordovician to early Silurian (430–457 Ma). By evaluating the congruence between the observed times of appearance of clade in the fossil record and the results derived from molecular data, we found that the uncorrelated rate model gave more congruent results for shallower divergence events whereas the auto-correlated rate model gives more congruent results for deeper events.