@[toc](Contents) ## Summary ## The fins-to-limbs transition was a major milestone in the history of life that shaped the morphology and remarkable biodiversity of land vertebrates. This project seeks to unravel the evolutionary changes in modularity of the musculoskeletal system that occurred during the evolution from fins to limbs and how these newly acquired modular organizations facilitated the evolution of different morphologies for the forelimb and hindlimb. ## Aim & Objectives ## The major aim of this project is to use anatomical network analyses of limbs to study the fin-limb transition, and specifically to unravel (1) the evolutionary changes in modularity of the musculoskeletal anatomy of fins and (2) how these newly acquired modular organizations might have facilitated the evolution of different morphologies for the fore- and hindlimbs in modern tetrapods. The objectives of this project are: - To gather musculoskeletal data from extant species. - To construct muscle attachments in extinct species spanning the fin-limb transition. - To build anatomical network models of limbs. - To identify musculoskeletal modules and measure the strength of their integration. ![enter image description here][1] ## Hypotheses ## **H1: The fore- and hindlimbs have less similar modularity in sarcopterygian fishes than in early tetrapods, and more similarity in the latter than in modern tetrapods.** According to the similarity bottleneck hypothesis, we argue that the fore- and hindlimbs have a more similar modularity among Devonian tetrapods than among sarcopterygian fishes and post-Devonian tetrapods, because morphological changes also caused topological changes in the limbs’ musculoskeletal anatomy. Rejecting H1 means that the limbs’ modularity evolved independently of other morphological features. **H2: The modularity of the fore- and hindlimbs evolved in a stepwise fashion across the fin-limb transition, from patterns resembling those of sarcopterygian fishes to approach those of modern tetrapods.** Limb morphology evolved a variety of functional adaptations in tetrapods after the fin-limb transition; we argue that this sequential adaptation to different land environments was accompanied by modification of their morphological modularity, because of the coordination of morphological changes. Rejecting H2 means that the modification of modularity is not required to permit independent morphological adaptive changes in the fore- and hindlimbs. **H3: Bones and muscles show a de-coupled modularity response to the new functional demands imposed during the fin-limb transition.** Although coordinated morphological responses are necessary for organisms to adapt to new conditions, we argue that bone and muscle networks experienced independent modifications of modularity, which we will identify as a significant de-coupling in the results of H1 and H2 in bone vs. muscle networks, because bones and muscles respond differentially in time and magnitude to evolutionary pressures. Rejecting H3 means that bones and muscles had a strong evolutionary coordination during the fin-limb transition and further locomotor adaptation. ## Methods ## - Anatomical dissections and developmental studies of extant species assisted by literature. - Reconstructing musculoskeletal anatomy in extinct species: 1. Bone geometry acquisition by digitization and image processing from micro-CT scans. 2. Muscle attachments reconstruction using the Extant Phylogenetic Bracket method. - Modeling anatomical networks of limbs and identifying connectivity modules. A network model is a mathematical formalization of the relations among a system’s constitutive parts. In an anatomical network of limbs, nodes of the network represent bones and muscles of the limb, while links represent articulations and attachments among them. A connectivity module is as a group of anatomical parts with more connections among them than to other parts outside the group. The topological arrangement of bones and muscles in each limb network determines connectivity modules, which represent organizational morphological modules. [1]: https://mfr.osf.io/export?url=https://osf.io/p6d4z/?action=download&mode=render&direct&public_file=False&initialWidth=848&childId=mfrIframe&parentTitle=OSF%20%7C%20Figure%20Abstract%20H2020%20report.png&parentUrl=https://osf.io/p6d4z/&format=2400x2400.jpeg