Understanding how extinct mammals moved provides essential clues about their ecological roles and adaptive strategies. In mammals, the astragalus (ankle bone) is crucial for locomotion because it transfers forces between the leg and the foot, balancing stability and mobility during walking. This study focuses on the examination of the astragali of two extinct South American mammals, Caraguatypotherium munozi and Trachytherus spegazzinianus, both members of Mesotheriidae—a group traditionally considered fossorial generalists. By combining bone measurements with functional indices, the performance of the ankle during the stance phase of walking was estimated. The results reveal marked ecological differences: C. munozi occupied a terrestrial–scansorial niche, showing moderate joint mobility and mechanical efficiency consistent with climbing tendencies, whereas T. spegazzinianus exhibited a semifossorial profile with a more stability-oriented ankle suited for digging. Despite these ecological differences, both species shared similar stance-phase kinematic ranges, reflecting a conserved plantigrade locomotor module optimized for stability and energy-efficient weight support. These findings challenge the traditional view of mesotheriids as functionally uniform and demonstrate that closely related species could achieve ecological diversification through subtle morphological adjustments within a shared locomotor framework.