Function and Constraint in the Marsupial Postcranium

The evolution of marsupial postcranial diversity and adaptation has long been conceptually tied to the ability of the otherwise highly immature neonates to actively move to the mother’s pouch after birth. This requirement is reflected in an unusually well-developed forelimb and anterior postcranial skeleton, which gave rise to the long-standing contention that marsupial postcranial evolution is under a developmental diversity constraint. In this chapter, the knowledge about early developmental processes and heterochrony behind marsupial postcranial development is summarized. This is followed by a discussion of recent finds that do not support the constraints hypothesis, arguing that these shine a new light on the usefulness of marsupial postcranial development in the study of vertebrate postcranial evo-devo. Australian marsupial postcranial diversity in particular is an excellent opportunity to study adaptations to the most common (and in the case of kangaroos, most specialized) locomotor modes in an old and isolated radiation of mammals. This topic is reviewed by providing an overview over the state of knowledge on the function and adaptation of the musculoskeletal system within the main locomotor categories of Australian marsupial mammals: generalized terrestrial quadrupeds, arboreal/scansorial species, gliders, fossorial species, and saltators (hoppers).