Hanging on and digging deep: comparative forelimb myology of the koala (Phascolarctos cinereus) and common wombat (Vombatus ursinus)

Establishing relationships between soft tissues and bones in living species can inform our understanding of functional adaptations in their extinct kin in the absence of direct data on habitual behaviours. The koala and the wombats are the only surviving species of the vombatiform suborder, and represent the extant phylogenetic bracket for dozens of fossil marsupial species across nine families. Here we present the first quantitative architectural data on the forelimb myology of the koala and common wombat, alongside redescriptions based on physical and digital dissections. Despite obvious contrasts in how their forelimbs are used in arboreal (koala) and fossorial (wombat) functional contexts, overall they showed only minor differences in qualitative and quantitative measures of myology. When the architectural properties of antagonistic pairs of muscle groups crossing each joint were compared, the greatest contrasts were seen in muscles crossing the elbow, where the wombat exhibited greater relative emphasis on forceful elbow extension than the koala, indicative of adaptations for digging. Our findings reinforce the importance of the elbow as a useful system to explore when seeking discrimination between habitual patterns of forelimb use in extinct vombatiform species.