Anatomical Network Analyses of Pre- and Post-Metamorphic Cranial Systems Support the Adaptive Decoupling Hypothesis in Anura

Complex life cycles characterized by distinct life phases connected by a metamorphosis raise the question whether traits of these distinct life phases can evolve independently or remain evolutionarily coupled. Because metamorphosis can include trait reorganization between life phases, the adaptive decoupling hypothesis proposes that larval and adult traits may evolve largely independent of each other. In anurans, the cranial system undergoes a major reorganization during metamorphosis, transitioning from a distinct cartilaginous larval cranium to an ossified adult cranium, yet this transformation has never been quantitatively characterized across species. We applied Anatomical Network Analysis (AnNA) to the cranial musculoskeletal systems of larval and adult anurans with different feeding and developmental modes. AnNA describes the topological organization of the cranium without the need for homologous landmarks, allowing comparisons between morphologically disparate systems. We calculated topological parameters describing integration and modularity, as well as connectivity modules (Q-modules) within each cranial network. As part of that analysis, expanded morphological descriptions of the cranial musculoskeletal system of larval and adult Smilisca phaeota were made. Larval and embryonic crania are topologically highly integrated and less modular, whereas adult crania are less integrated and organized into distinct modules. As a result, larval and adult life phases occupy distinct regions within a topological morphospace, whereby larvae and embryos of endotrophic anurans, despite lacking a free-living larval stage, cluster consistently within the larval topological space. Together, these findings support the adaptive decoupling hypothesis for the cranial system across anuran life phases.