1 Timing the extant avian radiation: The rise of modern birds, and the importance of modeling molecular rate variation Daniel J. Field1, Jacob S. Berv2, Allison Y. Hsiang3, Robert Lanfear4, Michael J. Landis5, Alex Dornburg6 1 Department of Earth Sciences, University of Cambridge, Cambridge, Cambridgeshire, United Kingdom 2Department of Ecology & Evolutionary Biology, Cornell University, Ithaca, New York, USA 3Department of Bioinformatics and Genetics, Swedish Museum of Natural History, Stockholm, Sweden 4Ecology and Evolution, Research School of Biology, Australian National University, Canberra, Australia 5Department of Ecology & Evolutionary Biology, Yale University, New Haven, Connecticut, USA 6 North Carolina Museum of Natural Sciences, Raleigh, North Carolina, USA Corresponding author: Daniel J. Field1 Email address:
[email protected] PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27521v1 | CC BY 4.0 Open Access | rec: 6 Feb 2019, publ: 6 Feb 2019 2 ABSTRACT Unravelling the phylogenetic relationships among the major groups of living birds has been described as the greatest outstanding problem in dinosaur systematics. Recent work has identified portions of the avian tree of life that are particularly challenging to reconstruct, perhaps as a result of rapid cladogenesis early in crown bird evolutionary history (specifically, the interval immediately following the end-Cretaceous mass extinction). At face value this hypothesis enjoys support from the crown bird fossil record, which documents the first appearances of most major crown bird lineages in the early Cenozoic—in line with a model of rapid post-extinction niche filling among surviving avian lineages. However, molecular-clock analyses have yielded strikingly variable estimates for the age of crown birds, and conflicting inferences on the impact of the end-Cretaceous mass extinction on the extant bird radiation.