Evolution of tooth number and size reveals random evolution and weak directional selection of dentition toward independent tooth loss in Mesozoic birds

How their toothed jaws evolve to edentulous beaks remains a key puzzle in the early birds. Although some studies using discrete data of dentition have rejected the hypotheses of long-term directional selection or weight-saving adaptations, a more detailed evolutionary pattern of dentition reduction is poorly quantified. Here for the first time, we analyze four essential parameters including tooth number on premaxilla, maxilla, and dentary, and average tooth crown size across a phylogeny of 21 key genera in Mesozoic birds. Our phylogenetic comparative analyses reveal that the evolution of tooth number is best described by the Brownian motion (BM) model, and this evolution is a random process driven primarily by time and phylogeny, with no evidence for a global trend towards tooth loss. However, at the same time, the evolution of tooth crown size shows a weak but detectable signal of directional selection (Ornstein-Uhlenbeck model) towards a smaller teeth and independent of body size, which may reflect an evolutionary transition of jaw function or feeding behavior. Furthermore, we quantify that the complete tooth loss occurred independently in at least four lineages (e.g., Confuciusornis , Gobipteryx , Archaeorhynchus , Schizooura ), and the key ancestral nodes are not edentulous. Lastly, we conclude that the predominant mode of dentition evolution in Mesozoic birds is random. The localized and independent events of tooth loss are likely driven by the lineage-specific ecological or developmental factors. Furthermore, the origin of modern bird-like digestive system, rather than the strong selective pressure, should be the key biological trigger to the tooth loss.