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Biol Lett. 2016 Nov;12(11). pii: 20160609. doi: 10.1098/rsbl.2016.0609.

Probabilistic divergence time estimation without branch lengths: dating the origins of dinosaurs, avian flight and crown birds.

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Department of Biological Sciences, Macquarie University, New South Wales 2109, Australia
Department of Geology, University of California Davis, One Shields Avenue, Davis, CA 94568, USA.
Department of Geology and Geological Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA.
Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK.
Department of Biology, University of York, Wentworth Way, York YO10 5DD, UK.


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 with 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: (i) Dinosauria, (ii) Avialae (the earliest birds) and (iii) Neornithes (crown birds). We find: (i) the plausibility of a Permian origin for dinosaurs to be dependent on whether Nyasasaurus is the oldest dinosaur, (ii) a Middle to Late Jurassic origin of avian flight regardless of whether Archaeopteryx or Aurornis is considered the first bird and (iii) a Late Cretaceous origin for Neornithes that is broadly congruent with other node- and tip-dating estimates. Demonstrating the feasibility of probabilistic time-scaling further opens up divergence estimation to the rich histories of extinct biodiversity in the fossil record, even in the absence of detailed character data.


birds; dinosaurs; divergence time; phylogeny

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