Recently, comprehensive morphological datasets including nearly all the 1 5 well-recognized Mesozoic birds become available, making it feasible for statistically 1 6rigorous methods to unveil finer evolutionary patterns during early avian evolution.
7However, few quantitative and statistical studies have yet been performed. Here, we 1 8 exploited the advantage of Bayesian tip dating under relaxed morphological clocks to 1 9 infer both the divergence times and evolutionary rates while accounting for their 2 0uncertainties. We further subdivided the characters into six body regions (i.e., skull, 2 1 axial skeleton, pectoral girdle and sternum, forelimb, pelvic girdle, and hindlimb) to 2 2 assess evolutionary rate heterogeneity both along the lineages and across partitions.
3We observed extremely high rates of morphological character changes during early 2 4 avian evolution and the clock rates are quite heterogeneous among the six regions.
5The branch subtending Pygostylia shows extremely high rate in the axial skeleton, 2 6 while the branches subtending Ornithothoraces and Enantiornithes show very high 2 7rates in the pectoral girdle and sternum, and moderately high rates in the forelimb.
8The extensive modifications in these body regions largely correspond to refinement of 2 9 the flight capability. The rest of the relatively slow and even rates suggest that there is 3 0 no dominant selective pressure in favoring of modifications in the skull and pelvis. 3 1 This study reveals the power and flexibility of Bayesian tip dating implemented in 3 2MrBayes to investigate evolutionary dynamics in deep time. 3 3 3 4