Soft tissue constraints on hip joint mobility in a sprawling and parasagittal amniote

Soft tissues, in addition to bones, are known to constrain joint mobility, yet the amount and type of influence remain underexplored. To investigate this further, we used XROMM to quantify ex vivo hip joint mobility during sequential removal of soft tissue layers (e.g. integument, extrinsic muscles, intrinsic muscles) in two amniotes that span a postural continuum: the sprawling tegu lizard (Salvator merianae) and the parasagittal Virginia opossum (Didelphis virginiana). We then compared ex vivo hip joint mobility spaces with in vivo joint excursions during steady-state walking recorded with bi-planar fluoroscopes. Our results demonstrate that integument passively restricts hip joint mobility in the tegu but not the opossum, while extrinsic muscles restrict mobility in the opossum but not the tegu due to the relatively bulky thighs of mammals. We further show that the tegu has greater ex vivo mobility than the opossum across tissue layers and during in vivo walking despite the classic ball-and-socket hip joint structure of mammals - a morphology traditionally interpreted as highly mobile. Finally, comparing in vivo hip joint excursion during walking with ex vivo mobility in both animals showed that soft tissues restrict stance phase excursions while swing phase movements approach the edge of viable ex vivo pose space. Collectively, our experimental findings provide novel insights and expand our understanding of the influence of soft tissues on joint mobility in vivo, ex vivo and in animals with unique postures, helping to aid future reconstructions of joint function and movement in extinct vertebrates.