The coevolution of rostral keratin cover and toothrow distribution in Mesozoic dinosaurs

Teeth evolved early in vertebrate evolution, and their morphology reflects
important specializations in diet and ecology among species. The toothless
jaws (edentulism) in extant birds likely coevolved with beak keratin,
which functionally replaced teeth. However, extinct dinosaurs lost teeth
multiple times independently and exhibited great variation in toothrow
distribution and beak-like keratin structures. Here, we use facial jawbone
surface texture as a proxy for rostral keratin covering and phylogenetic
comparative models to test for the influence of facial keratin on toothrow
distribution in Mesozoic dinosaurs. We find that the evolution of rostral
keratin covering explains partial toothrow reduction but not jaw
toothlessness. Toothrow reduction preceded the evolution of rostral
keratin cover in theropods. Non-theropod dinosaurs evolved continuous
toothrows despite rostral keratin cover (e.g., some ornithischians and
sauropodomorphs). We also show that rostral keratin cover did not
significantly increase the evolutionary rate of tooth loss, which further
delineates the antagonistic relationship between these structures. Our
results suggest that the evolution of rostral keratin had a limited effect
on suppressing tooth development. Independent changes in jaw development
may have facilitated further tooth loss. Furthermore, the evolution of
strong chemical digestion, a gizzard, and a dietary shift to omnivory or
herbivory likely alleviated selective pressures for tooth
development.