A study identifies links between beak evolution and loss of dentition in two lineages of theropod dinosaurs. Numerous theropod dinosaur lineages, including birds, lost their teeth as they evolved. However, the developmental mechanisms linking the evolutionary loss of teeth and beak formation remain unresolved. Shuo Wang and colleagues examined jaw fossils from one of the smallest known dinosaurs from the group Caenagnathidae and the early Cretaceous bird Sapeornis. Computer tomography and synchrotron scans indicated the presence of vestigial bony tooth sockets and other jaw morphologies similar to another beaked theropod, Limusaurus. Previous research indicates that Limusaurus transitioned from toothed juveniles to toothless beaked adults, suggesting that a reduction of dentition during maturation may explain the similar dental morphologies of the three theropods. Character correlation analyses indicate that terrestrial egg laying, the presence of keratinous egg teeth, and the acquisition of toothless beaks are correlated in tetrapods, potentially explaining the propensity for beak evolution in Archosauromorpha, which includes birds, crocodiles, and pterosaurs. The authors suggest that a protein called bone morphogenetic protein 4 may have simultaneously mediated embryonic tooth loss and beak keratin formation. According to the authors, the evolutionary progression of postnatal and embryonic dental reduction may have driven the repeated evolution of beaks in nonavian theropods and birds as a response to diet specialization.
Article #17-08023: "Heterochronic truncation of odontogenesis in theropod dinosaurs provides insight into the macroevolution of avian beaks," by Shuo Wang et al.