Cranial Asymmetry in Hyloidea (Anura, Neobatrachia): Patterns of Morphological Variation

Asymmetry in a bilateral organism refers to the difference in the expression of a trait between right and left sides, which may result from genetic or environmental disturbances. Using geometric morphometrics and a phylogenetic approach, we studied asymmetry in dorsal and ventral views of the skulls of anuran species, representing 22 families, 16 of which belong to the Hyloidea clade. The cranial regions with the most pronounced shape asymmetry were identified. To discern species with elevated levels of asymmetry and to hypothesize its evolutionary trends across phylogeny we implemented the asymmetry index. Significant asymmetric skull shape variation was found between right and left sides, associated with the upper area of the mandibular joint and the anterior area of the nasals in dorsal view. In ventral view, the greatest variation was in the vomers and the ala of the parasphenoid. The degree of size-related asymmetry varied among species. Character mapping results indicate that cranial asymmetry is a conserved and widespread trait in the Hyloidea clade, representing an ancestral condition across both dorsal and ventral regions of the skull. The observed asymmetries were found in traits that develop later in their ontogeny, which would imply that as they establish fewer developmental dependencies with other traits and, consequently, are less phylogenetically constrained. The asymmetries we found belong to this category and would be more prone to change during evolution.