Anatomical network analysis of the musculoskeletal system reveals integration loss and parcellation boost during the fins-to-limbs transition

Tetrapods evolved from within the lobe-finned fishes around 370 Ma. The evolution of limbs from lobe-fins entailed a major re-organization of the skeletal and muscular anatomy of appendages in early tetrapods. Concurrently, a degree of similarity between pectoral and pelvic appendages also evolved. Here, we compared the anatomy of appendages in extant lobe-finned fishes (Latimeria and Neoceratodus) and anatomically plesiomorphic amphibians (Ambystoma, Salamandra) and amniotes (Sphenodon) to trace and reconstruct the musculoskeletal changes that took place during the fins-to-limbs transition. We quantified the anatomy of appendages using network analysis. First, we built network models—in which nodes represent bones and muscles, and links represent their anatomical connections—and then we measured network parameters related to their anatomical integration, heterogeneity, and modularity. Our results reveal an evolutionary transition toward less integrated, more modular appendages. We interpret this transition as a diversification of muscle functions in tetrapods compared to lobe-finned fishes. Limbs and lobe-fins show also a greater similarity between their pectoral and pelvic appendages than ray-fins do. These findings on extant species provide a basis for future quantitative and comprehensive reconstructions of the anatomy of limbs in early tetrapod fossils, and a way to better understand the fins-to-limbs transition.