Here, we assemble a novel phylogenetic hypothesis for Mesozoic dinosaurs and time-scale it using two different probabilistic APT methods-cal3 and a new method developed from the node-dating approach of Hedman. While Bayesian tip-dating methods have recently become accessible to completely extinct clades ( other papers in Special Feature), their availability has also coincided with improvements in APT approaches. These approaches suffer from arbitrary choices of required variables and make strong assumptions of the quality of the fossil record without reference to said fossil record. The latter two utilize a minimum age (based on first appearances) dated tree as a preliminary step. Broadly speaking, three types are typically applied: (i) minimum-age dating, (ii) extending branch durations by adding a constant and (iii) branch duration sharing. Most of these APT approaches work independently of inferred amounts of character change (but see ), relying solely on occurrence data. We term these ‘ a posteriori’ time-scaling (APT) approaches. Often constructed from both morphological cladograms and taxonomy, these lack measures of character change, with their strength instead relying on the stratigraphic distribution of fossils.

Independently of these advancements, palaeontologists have been using stratigraphic ages to directly date divergences on existing phylogenies. This opens up the possibility for using character change from molecular or morphological sources (or both) when estimating divergences between extinct and extant lineages, or even among entirely extinct lineages. More recently, ‘tip-dating’ approaches have allowed extinct taxa to be included as terminals, with phylogenetic inference and divergence time estimation occurring simultaneously without reference to node calibrations. origin of birds) and thus cannot be estimated using molecular data.

However, multiple important divergences within the tree of life are not bracketed by extant lineages (e.g. In a classical clock-based ‘node-dating’ framework, a tree of extant taxa with branch lengths representing character changes is dated with reference to a set of fossil calibrations that constrain the minimum age for particular nodes. 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.ĭivergence times are often estimated by combining fossil information with a phylogenetic hypothesis. 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. 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). Among these methods, sophisticated probabilistic approaches have recently emerged, in contrast with simpler algorithms relying on minimum node ages. However, a separate set of divergence time approaches are typically used to date palaeontological trees, which may lack such branch lengths. 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.

Estimated flight durations