LAUSR

Abstract A paddle-shaped wing, the general morphology of the humerus, and the muscles involved in wing movement are among the most characteristic adaptations to diving in penguins. Particularly, the humeral fossa tricipitalis and the musculus humerotriceps are clear examples of muscular rearrangement accompanying skeletal changes. In extant Spheniscidae, we were able to identify two heads of this muscle attaching within a different compartment of the bipartite fossa. Since the partition of the fossa appeared as a novelty during the Miocene, we propose that this might have had implications for underwater flight contributing to wing-propelled diving efficiency.