Kinematic comparisons between mudskipper fins and salamander limbs during terrestrial locomotion.

Terrestrial and aquatic environments present drastically different challenges, yet amphibious behaviors evolved multiple times in vertebrates. Terrestrial salamanders are often used to model the locomotion of crownward stem tetrapods, but amphibious fishes may model earlier evolutionary stages as vertebrates became terrestrial. For instance, some early tetrapods may have moved on land with a mudskipper-like gait. Previously published kinetic data found that the ground reaction forces produced by the pectoral fins of mudskippers (Periophthalmus barbarus) were more medial than the limbs of tiger salamanders (Ambystoma tigrinum), which might elevate bending stresses in the fins. However, kinematic data are needed to explain these kinetic differences. Therefore, we quantified the three-dimensional kinematics of mudskipper pectoral fins and compared these to published data on tiger salamander limbs. We found that mudskipper pectoral fins generally remained more retracted, extended, and adducted compared to salamander limbs. Kinematic patterns in mudskipper pectoral fins were aligned with published kinetic data and shared a restricted range of motion found in early tetrapods. Our findings demonstrate that mudskipper pectoral fins provide weight support and propulsion but have lower mobility in the proximal versus distal elements, for which greater flexibility in the latter might compensate. Broadly, these data provide new insights into the biomechanics of using fins versus limbs for moving over land and factors that may favor the evolution of different terrestrial gaits.