Branch lengths – measured in character changes – are an essential requirement of clock-based divergence estimation, regardless of whether the fossil calibrations used represent nodes or tips. However, a separate set of divergence time approaches are typically used to date palaeontological trees, which may lack such branch lengths. Among these methods, sophisticated probabilistic approaches have recently emerged, in contrast to simpler algorithms relying on minimum node ages. Here, using a novel phylogenetic hypothesis for Mesozoic dinosaurs, we apply two such approaches to estimate divergence times for: 1) Dinosauria, 2) Avialae (the earliest birds), and 3) Neornithes (crown birds). We find: 1) the plausibility of a Permian origin for dinosaurs to be contingent on the phylogenetic position of Nyasasaurus, 2) a Middle to Late Jurassic origin of avian flight regardless of whether Archaeopteryx or Aurornis is considered the first bird, and 3) a Late Cretaceous origin for Neornithes that is broadly congruent with other node- and tip-dating estimates. Demonstrating the feasibility of probabilistic time-scaling opens up divergence estimation to the rich histories of extinct biodiversity in the fossil record, even in the absence of detailed character data.
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