The vertebrate head is an integrated system essential for sensory functions, capturing prey, and defense mechanisms. Head anatomy has long attracted the attention of biologists, yet identifying the factors responsible for the evolution of deviant morphological forms has remained a long-standing challenge. Frogs are one of the most diverse vertebrate orders but have not been thoroughly studied with respect to cranial morphological variation. We use extensive sampling of all major lineages to quantify skull diversity, reconstruct the evolution of increased mineralization (hyperossification), and test for relationships between ecology, skull shape, and hyperossification. We find that several extreme skull shapes have repeatedly evolved in frogs, hyperossification has arisen independently many times, and deviant skulls often cooccur with hyperossification and specialized functions.
Frogs (Anura) are one of the most diverse vertebrate orders, comprising more than 7,000 species with a worldwide distribution and extensive ecological diversity. In contrast to other tetrapods, frogs have a highly derived body plan and simplified skull. In many lineages of anurans, increased mineralization has led to hyperossified skulls, but the function of this trait and its relationship with other aspects of head morphology are largely unexplored. Using three-dimensional morphological data from 158 species representing all frog families, we assessed wide-scale patterns of shape variation across all major lineages, reconstructed the evolutionary history of cranial hyperossification across the anuran phylogeny, and tested for relationships between ecology, skull shape, and hyperossification. Although many frogs share a conserved skull shape, several extreme forms have repeatedly evolved that commonly are associated with hyperossification, which has evolved independently more than 25 times. Variation in cranial shape is not explained by phylogenetic relatedness but is correlated with shifts in body size and ecology. The species with highly divergent, hyperossified skulls often have a specialized diet or a unique predator defense mechanism. Thus, the evolution of hyperossification has repeatedly facilitated the expansion of the head into multiple new shapes and functions.