An Eocene Origin of Passerine Birds Estimated Using Bayesian Tip Dating with Fossil Occurrences

Passerine birds are among the most diverse and species-rich groups of vertebrates, but the timescale of their evolution has been difficult to resolve with confidence. The fossil record of early passerines is relatively sparse and molecular-clock estimates of the passerine crown age have varied widely, with most previous studies relying on external fossil calibrations or assumptions relating to Gondwanan vicariance. In this study, we estimated the passerine evolutionary timescale by incorporating a set of 43 passerine fossils selected through a detailed assessment, while using a Bayesian tip-dating approach with the unresolved fossilized birth-death process. Our analyses ultimately place the passerine crown age in the Eocene, which largely closes the gap between molecular and palaeontological estimates of the passerine evolutionary timescale. Our date estimates are somewhat influenced by the prior probability density for the starting time of the diversification process. Through a simulation study, we show that the effect of the starting-time prior can be attenuated by the inclusion of morphological data for fossil and extant taxa. Overall, our study demonstrates that incorporating a curated, comprehensive set of fossils is effective in producing a well-resolved estimate of the passerine evolutionary timescale, while highlighting potential avenues for refining this estimate using Bayesian tip-dating analyses.