LAUSR

Hydrofoil‐shaped limbs (flipper‐hydrofoils) have evolved independently several times in secondarily marine tetrapods and generally fall into two functional categories: (1) those that produce the majority of thrust during locomotion (propulsive flipper‐hydrofoils); (2) those used primarily to steer and resist destabilizing movements such as yaw, pitch, and roll (controller flipper‐hydrofoils). The morphological differences between these two types have been poorly understood. Theoretical and experimental studies on engineered hydrofoils suggest that flapping hydrofoils with a flexible trailing edge are more efficient at producing thrust whereas hydrofoils used in steering and stabilization benefit from a more rigid one. To investigate whether the trailing edge is generally more flexible in propulsive flipper‐hydrofoils, we compared the bone distribution along the chord in both flipper types. The propulsive flipper‐hydrofoil group consists of the forelimbs of Chelonioidea, Spheniscidae, and Otariidae. The controller flipper‐hydrofoil group consists of the forelimbs of Cetacea. We quantified bone distribution from radiographs of species representing more than 50% of all extant genera for each clade. Our results show that the proportion of bone in both groups is similar along the leading edge (0–40% of the chord) but is significantly less along the trailing edge for propulsive flipper‐hydrofoils (40–80% of the chord). Both flipper‐hydrofoil types have little to no bony tissue along the very edge of the trailing edge (80–100% of the chord). This suggests a relatively flexible trailing edge for propulsive flipper‐hydrofoils compared to controller flipper‐hydrofoils in line with findings from prior studies. This study presents a morphological correlate for inferring flipper‐hydrofoil function in extinct taxa and highlights the importance of a flexible trailing edge in the evolution of propulsive flipper‐hydrofoils in marine tetrapods.