Hop, walk or bound? Limb proportions in kangaroos and the probable locomotion of the extinct genus Protemnodon

Kangaroos (Macropodoidea) display a diversity of locomotor modes, from bounding quadrupedally to hopping bipedally; but hopping has a body mass limit, which was exceeded by a number of extinct taxa. In the Pleistocene, a variety of “giant” kangaroos existed, both within the extinct subfamily Sthenurinae and the extant subfamily Macropodinae (both within the family Macropodidae). Sthenurines have been previously considered to have a type of locomotion (bipedal striding) different from extant kangaroos, but the primary locomotor mode of the large species of the extinct “giant” macropodine genus Protemnodon, closely related to extant large kangaroos, has undergone little question and has been assumed to be hopping. Here, the association between limb proportions and locomotor mode across Macropodoidea is assessed by examination of functional limb indices. We show that large (> 100 kg) Protemnodon species are unlike any other known macropodoids; their position in this functional morphospace, along with previously published evidence on humeral morphology, supports a prior hypothesis of a primarily quadrupedal mode of locomotion, likely some sort of bounding.