The locomotor behavior of subfossil Malagasy sloth-lemurs (Strepsirrhini: Palaeopropithecidae) and koala-lemurs (Strepsirrhini: Megaladapidae): new insights from limb trabecular bone

The locomotion of Malagasy Quaternary subfossil lemurs—including palaeopropithecids (‘sloth-lemurs’) and megaladapids (‘koala-lemurs’)—has been investigated on abundant postcranial remains. Proposed strategies include some without living primate parallels, including sloth-like suspensory arboreality in palaeopropithecids, although the degree of suspensory behaviour in palaeopropithecids, or locomotor diversity in koala-lemurs are poorly understood. Differently from external morphology, internal bone structure in these taxa is largely unexplored. We compared the humeral and femoral trabecular architecture of sloth- and koala-lemurs to several extant mammals, studying spherical trabecular samples extracted from high-resolution scans. After defining locomotor categories from quantitative data, we tested links between trabecular parameters and locomotor modes through exploratory and multivariate analyses, accounting for body size and phylogeny. In extant mammals, only femoral trabecular traits—particularly Degree of Anisotropy and Bone Volume Fraction—were significantly associated with locomotion, distinguishing suspensory and bridging arboreal taxa from others. Using this model, we inferred suspensory adaptations in palaeopropithecids, especially Palaeopropithecus —confirming earlier reconstructions—but also in Megaladapis edwarsi , a striking result that would make M. edwarsi the largest mammal ever known to adopt such habits, approached only by extant orangutans. This work highlights the potential of internal bone structure for reconstructing primate locomotor evolution.