Bayesian tip dating reveals heterogeneous morphological clocks in Mesozoic birds

Abstract

Recently, comprehensive morphological datasets including nearly all the well-recognized Mesozoic birds become available, making it feasible for statistically rigorous methods to unveil finer evolutionary patterns during early avian evolution. However, few quantitative and statistical studies have yet been performed. Here, we exploited the advantage of Bayesian tip dating under relaxed morphological clocks to infer 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 extremely high rate in the axial skeleton, while the branches subtending Ornithothoraces and Enantiornithes show very 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. The rest of the relatively slow and even rates suggest that there is no dominant selective pressure in favoring of modifications in the skull and pelvis. This study reveals the power and flexibility of Bayesian tip dating implemented in MrBayes to investigate evolutionary dynamics in deep time.