Tip dating and Bayes factors provide insight into the divergences of crown bird clades across the end-Cretaceous mass extinction

Abstract

The origin of crown birds (Neornithes) remains contentious owing to conflicting divergence time hypotheses obtained from alternative sources of data. The fossil record suggests limited diversification of Neornithes in the Late Mesozoic and a substantial radiation in the aftermath of the Cretaceous-Palaeogene (K-Pg) mass extinction, approximately 66 Ma. Molecular clock studies, however, have yielded estimates for neornithine origins ranging from the Early Cretaceous (130 Ma) to less than 10 Myr before the K-Pg. We use Bayes factors to compare the fit of node ages from different molecular clock studies to an independent morphological dataset. Our results allow us to reject scenarios of crown bird origins deep in the Early Cretaceous, as well as an origin of crown birds within the last 10 Myr of the Cretaceous. The scenario best supported by our analyses is one where Neornithes originated between the Early and Late Cretaceous (ca 100 Ma), while numerous divergences within major neoavian clades either span or postdate the K-Pg. This study affirms the importance of the K-Pg on the diversification of modern birds, and the potential of combined-evidence tip-dating analyses to illuminate recalcitrant 'rocks versus clocks' debates.

Keywords: Bayes factor; bird; end-Cretaceous extinction; phylogeny; tip-dating.

Figure 1.

Schematic phylogeny illustrating interrelationships of the major avian clades under investigation; extant diversity following the International Ornithological Congress (IOC) World Bird List 13.2 [1] illustrated at nodes. Silhouettes modified from [2].

Figure 2.

Maximum clade credibility trees produced by tip dating analysis using the fossilized birth–death model, with the best fitting set of node age priors according to Bayes factors. Dots at nodes indicate the major clades. (a) Asteriornis is constrained to a position within crown Galloanserae, J&P node age priors; (b) Asteriornis is constrained to a position within crown Neognathae but outside crown Galloanserae, Jarvis node age priors.

Figure 3.

Illustrations of the node age prior probabilities found to fit the observed data best, and median inferred ages of the principal neornithine clades. Density distributions represent the shape of the node-age priors used in the fossilized birth–death analysis; dashed lines represent the median inferred age. (a) Asteriornis is constrained to a position within crown Galloanserae, J&P node age priors; (b) Asteriornis is constrained to a position within crown Neognathae but outside crown Galloanserae, Jarvis node age priors.