Postmetamorphic vertebral development in hylid and leptodactylid frogs: insights into its relationship to modularity and morphological disparity

The vertebral column is central to the functional and ecological diversity of vertebrates, yet ontogenetic changes in vertebral morphology and morphological integration remain poorly explored in anurans. Here, we used geometric morphometrics to analyse postmetamorphic vertebral shape in 13 species of Hylidae and Leptodactylidae. Our aims were to characterize the variation in shape, identify potential patterns and ontogenetic shifts of modular organization, and evaluate the morphological disparity across development. The variation in shape was quantified via principal component analysis based on dorsal and ventral views of the presacral vertebrae and sacrum and on dorsal and lateral views of the urostyle. Ontogenetic analyses revealed marked shape transformations: juveniles exhibited laterally expanded and anteroposteriorly compressed vertebrae, whereas adults showed lateral compression, enlarged sacral diapophyses, and a more pronounced urostyle ridge. Modularity analyses indicated a postmetamorphic ontogenetic shift, in which juveniles showed greater independence among vertebral structures, whereas adults exhibited stronger covariation than juveniles between vertebrae V–VI and the sacro-urostylar complex. Vertebra VIII displayed the highest morphological disparity, reflecting a substantial change in shape. Family-level differences were clear: Leptodactylidae displayed a robust atlas, compact vertebrae, prominent neural spines, and a vertebral column length equalling 61% of snout–vent length; in contrast, Hylidae had more anteroposteriorly compressed vertebrae, wider cotyle spacing, reduced neural spines, and a vertebral column measuring 67% of snout–vent length. Together, these findings expand our understanding of anuran vertebral development and provide a foundation for future evolutionary and functional studies.