From sagittal to sprawled: Clavicular co-option and the evolution of forelimb mechanics in talpines

The members of Eulipotyphla exhibit substantial ecological and locomotor diversity, yet forelimb skeletal correlates of these adaptations are not consistently shared across lineages. In particular, highly fossorial moles (Talpini and Scalopini) employ a distinctive lateral-thrust digging mechanism unlike that of most other fossorial mammals. We tested whether such differences are associated with variation in forelimb posture and its functional consequences. Using 31 extant specimens representing 22 species, we quantified the orientations of the scapula, clavicle, and humerus, and estimated the compressive strengths of the clavicle and first true rib based on three-dimensional models derived from computed tomography. Eulipotyphlans were divided into two groups based on humeral orientation: a "sagittal group" and a "sprawled group." While the first true rib strength scaled consistently with body mass (BM) across taxa, clavicular strength differed markedly between groups, being substantially greater in the "sprawled group." These results suggest that the first true rib plays a conserved role in transmitting vertical ground reaction forces, whereas the clavicle in talpines functions as a strut resisting medially directed forces under a sprawled posture. We further propose that increased clavicular strength preceded this postural transition and was subsequently co-opted (exaptation) to support lateral-thrust digging, functionally replacing the coracoid as a medial strut, as seen in monotremes.