Morphofunctional Spaces from the Astragalus: Exploring Angular Excursions and Mechanical Efficiency in...

The astragalus is a key biomechanical link between the autopodium and limb in mammals, modulating stability and mobility during stance. Its morphology provides reliable proxies for inferring posture, body mass, and locomotor behavior in extinct taxa. Mesotheriids (Notoungulata, Mesotheriidae) have traditionally been regarded as fossorial generalists, yet their functional diversity remains poorly tested. The astragali of Caraguatypotherium munozi (Miocene, Chile) and Trachytherus spegazzinianus (early Miocene, Argentina) were analyzed, integrating osteological measurements and functional indices to explore their locomotor ecology. Principal component analyses were performed to compare their morphofunctional spaces with those of 38 extant terrestrial mammals grouped by posture, body mass, top speed, and locomotor habit. Total Angular Excursion (TAE) and Angular Efficiency Index (AEI) were estimated using a comparative dataset of 182 terrestrial mammals spanning 15 taxonomic orders, focused on stance-phase mechanics during comfortable locomotion. C. munozi shows a deeper trochlea and moderately expanded navicular facets, whereas T. spegazzinianus presents a shallower trochlea and narrower articular proportions. Despite these morphological differences, both species share broadly overlapping stance-phase kinematic ranges, reflecting a conserved plantigrade locomotor module optimized for stability and energy-efficient weight support. These findings challenge the view of mesotheriids as functionally uniform and highlight ecological diversification through subtle morphological adjustments within a constrained locomotor framework.