Terrestrial Locomotion in Extant Marine Tetrapods with Flippers

The repeated evolutionary transitions of tetrapods from terrestrial to marine environments represent one of the most remarkable examples of functional convergence among vertebrates. Despite profound specialisation for aquatic locomotion, many marine tetrapods must periodically return to land for essential activities such as reproduction, moulting, and resting, providing a unique opportunity to investigate whether convergent adaptations for swimming have been accompanied by convergent evolution of terrestrial locomotor repertoires. This study combines a comparative synthesis with original phylogenetic analyses to investigate the evolution of terrestrial locomotor repertoires in the three principal lineages of extant marine tetrapods with flippers: sea turtles, penguins, and pinnipeds. A standardised behavioural dataset compiled from published studies, publicly available videos, and original observations was used to reconstruct the evolutionary history of locomotor behaviours, test correlated evolution among locomotor modes, and evaluate the influence of body mass within a phylogenetic framework. Although these lineages exhibit remarkable convergence in appendicular adaptations for aquatic propulsion, our comparative analyses demonstrate that this convergence has not been accompanied by convergence in terrestrial locomotor repertoires. Instead, each lineage has retained or evolved distinct terrestrial locomotor strategies reflecting different combinations of phylogenetic inheritance, biomechanical constraints, and ecological specialisation. Marine turtles exhibit the most conservative terrestrial repertoire, penguins combine highly specialised underwater flight with diverse terrestrial behaviours, and pinnipeds display the greatest diversity of terrestrial locomotor strategies. These findings indicate that similar functional demands for efficient swimming have generated multiple evolutionary solutions for terrestrial locomotion rather than a single convergent strategy. More broadly, this study provides an integrative framework for understanding how behaviour, biomechanics, ecology, and evolutionary history interact to shape locomotor evolution in secondarily aquatic tetrapods and establishes a behavioural foundation for future studies linking structure, performance, behaviour, and fitness.