Abstract
Recently, comprehensive morphological datasets including nearly all the well-recognized Mesozoic birds became available, making it feasible for statistically rigorous methods to unveil finer evolutionary patterns during early avian evolution. Here, we exploited the advantage of Bayesian tip dating under relaxed morphological clocks to estimate both the divergence times and evolutionary rates while accounting for their uncertainties. We further subdivided the characters into six body regions (i.e. skull, axial skeleton, pectoral girdle and sternum, forelimb, pelvic girdle and hindlimb) to assess evolutionary rate heterogeneity both along the lineages and across partitions. We observed extremely high rates of morphological character changes during early avian evolution, and the clock rates are quite heterogeneous among the six regions. The branch subtending Pygostylia shows an extremely high rate in the axial skeleton, while the branches subtending Ornithothoraces and Enantiornithes show notably high rates in the pectoral girdle and sternum and moderately high rates in the forelimb. The extensive modifications in these body regions largely correspond to refinement of the flight capability. This study reveals the power and flexibility of Bayesian tip dating implemented in MrBayes to investigate evolutionary dynamics in deep time.
Highlights
Birds are one of the most speciose and ecologically diverse living vertebrates [1]
In the Bayesian tip dating framework, we infer the posterior probability distribution of the model 3 parameters, which combines the information from the morphological characters and the priors
The fossilized birth-death (FBD) process [20,30,31,32] was used to model speciation, extinction, fossilization and sampling, which gave rise to the prior distribution of the time tree T, including the topology (τ) and branch lengths measured by Myr
Summary
Birds are one of the most speciose (over 10 000 recognized species) and ecologically diverse living vertebrates [1]. We further subdivided the characters into six anatomical regions to assess evolutionary rate heterogeneity across these regions: skull (53 characters), axial skeleton (36 characters), pectoral girdle and sternum (48 characters), forelimb (65 characters), pelvic girdle (23 characters) and hindlimb (55 characters) These partitions reflect relatively distinct body regions that have undergone different patterns of modification in the early phase of evolution. It has been productively applied to morphological data only [23,24,25,26] and we use ‘tip dating’ for that purpose This approach has the essential strengths of incorporating various sources of information from the fossil record directly in the analysis, modelling the speciation process explicitly through a probabilistic model, allowing for parameter inference and model selection and using the state-of-the-art developments in Bayesian computation
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