Unravelling drivers on the morphological diversification of the terminal phalanx in hyloid frogs

Abstract The anuran locomotor system integrates traits that are influenced by phylogenetic, ecological, and development constraints. Given their significance to locomotion, we studied terminal phalange morphology in the Hyloidea group. We aim to deduce if morphological variability stems from phylogenetic, ecological, or life-cycle constraints. We explore the influence of size on variation and assess if evolutionary rates and shape disparities differ among the groups under consideration. Finally, we optimized phalangeal morphology within the phylogenetic framework to delineate evolutionary trends. We included 424 specimens of 128 species representing 17 families of Hyloidea and two of non-hyloid anurans. Configuration of the terminal phalanx was quantified using geometric morphometrics and characterized through qualitative traits. We established four categories based on microhabitats and locomotor abilities. Our life-cycle categorization distinguishes species by their consistent or changing microhabitat across larval and adult stages. The results show a complex scenario, where certain clades occupy distinct regions of morphospace, but there is also a relationship between phalangeal shape, microhabitats, and locomotor abilities. However, both the phylogenetic signal and the relationship with microhabitats and locomotor abilities are not particularly robust. Species inhabiting arboreal microhabitats develop convergent traits to thrive in this niche, such as rounded proximal epiphysis and the claw-shaped phalanx. Morphological disparity was higher in walkers, which also includes arboreal species, prompting further questions on the demands of locomotion in vertical substrates.