Terrestrial force production by the limbs of a semi-aquatic salamander provides insight into the evolution of terrestrial locomotor mechanics

Amphibious fishes and salamanders are valuable functional analogs for vertebrates that spanned the water-land transition. However, investigations of walking mechanics have focused on terrestrial salamanders and, thus, may better reflect the capabilities of stem tetrapods that were already terrestrial. The earliest tetrapods were likely aquatic, so salamanders that are not primarily terrestrial may yield more appropriate data for modeling the incipient stages of terrestrial locomotion. In the present study, locomotor biomechanics were quantified from semi-aquatic Pleurodeles waltl, a salamander that spends most of its adult life in water, and then compared with those of a primarily terrestrial salamander (Ambystoma tigrinum) and a semi-aquatic fish (Periophthalmus barbarus) to evaluate whether terrestrial locomotion was more comparable between species with ecological versus phylogenetic similarities. Ground reaction forces (GRFs) from individual limbs or fins indicated that the pectoral appendages of each taxon had distinct patterns of force production, but GRFs from the hindlimbs were comparable between the salamander species. The rate at which force is produced can affect musculoskeletal function, so we also calculated 'yank' (first time derivative of force) to quantify the dynamics of GRF production. Yank was sometimes slower in P. warn but there were some similarities between the three species. Finally, the semi-aquatic taxa (P. waft/and P. barbarus) had a more medial inclination of the GRF compared to terrestrial salamanders, potentially elevating bone stresses among more aquatic taxa and limiting their excursions onto land.

Automated summary

This study quantifies the locomotor biomechanics of semi-aquatic salamanders and a semi-aquatic fish and compares them with those of a terrestrial salamander, evaluating the comparability of terrestrial locomotion between species with ecological and phylogenetic similarities. The study finds distinct patterns of force production in the pectoral appendages of different species, but comparable force production patterns in the hindlimbs. The study also suggests that semi-aquatic taxa have a greater inclination of ground reaction forces compared to terrestrial salamanders, potentially increasing bone stresses and limiting their excursions onto land.