Changes in pectoral appendicular muscle anatomy and function over the water‐land transition in tetrapods

The question of how tetrapod limbs evolved from fins is one of the great puzzles of vertebrate biology. While paleontologists, developmental biologists, and geneticists have made great strides in explaining the origin and early evolution of limb skeletal structures, that of the muscles remains largely unknown. We sought to fill this gap by reconstructing pectoral appendicular muscles in three fossil taxa and measuring changes in muscle leverage about the shoulder and elbow joints over the fin‐limb and water‐land transitions. Building upon hypotheses of muscle homology recently published by our group and osteological correlates from the literature, we used extant phylogenetic bracketing and character optimization to reconstruct pectoral appendicular muscles in fossil taxa spanning the fin‐limb and water‐land transitions: the tetrapodomorph fish Eusthenopteron and the stem tetrapods Acanthostega and Pederpes. Then, we built 3D biomechanical models of the three fossil taxa, plus an extant lungfish and an extant coelacanth to compare the leverage of various muscle groups over a range of limb postures. Our preliminary results support the idea that one of the first major functional changes in the ancestors of tetrapods was the division of the superficial dorsal muscle mass of the shoulder into deltoid and latissimus dorsi muscles. This division would have provided leverage for forelimb protraction and retraction over a larger range of motion, likely facilitating tetrapod‐like gaits. Support or Funding Information AAA Post‐doctoral Fellowship to J. Molnar