Abstract
Sthenurine kangaroos, extinct “giant kangaroos” known predominantly from the Plio-Pleistocene, have been proposed to have used bipedal striding as a mode of locomotion, based on the morphology of their hind limbs. However, sthenurine forelimb morphology has not been considered in this context, and has important bearing as to whether these kangaroos employed quadrupedal or pentapedal locomotion as a slow gait, as in extant kangaroos. Study of the correlation of morphology of the proximal humerus in a broad range of therian mammals shows that humeral morphology is indicative of the degree of weight-bearing on the forelimbs during locomotion, with terrestrial species being distinctly different from arboreal ones. Extant kangaroos have a proximal humeral morphology similar to extant scansorial (semi-arboreal) mammals, but sthenurine humeri resemble those of suspensory arboreal taxa, which rarely bear weight on their forelimbs, supporting the hypothesis that they used bipedal striding rather than quadrupedal locomotion at slow gaits. The humeral morphology of the enigmatic extinct “giant wallaby,” Protemnodon, may be indicative of a greater extent of quadrupedal locomotion than in extant kangaroos.
Highlights
The familiar locomotor mode of kangaroos is hopping: that is, bipedal locomotion using the hind limbs in tandem, unaided by the forelimbs
Our interest is in the functional morphology of the proximal humerus rather than its precise anatomy: we propose that the relative sizes of the greater tuberosity, lesser tuberosity, and humeral head are a direct proxy of humeral function
All of our analyses show that sthenurine kangaroos have a very different morphology of the proximal humerus from that seen in other macropodids, extant and extinct: rather, their morphology resembles that of extant mammals that engage in suspensory climbing
Summary
The familiar locomotor mode of kangaroos is hopping: that is, bipedal locomotion using the hind limbs in tandem, unaided by the forelimbs. At the slower speeds that make up the majority of the daily locomotor repertoire of kangaroos, some sort of forelimb-supported gait is employed, and this locomotion dominates the pattern of daily movement (see Doube et al 2018). Sthenurines are first known from the middle Miocene, and were always relatively large animals for their time, truly “giant” forms (i.e., larger than extant kangaroos) are not known until the Plio-Pleistocene. Plio-Pleistocene sthenurines comprised three to five genera and ~25 species (Prideaux 2004), with body mass estimates of 43–244 kg (Helgen et al 2006). Species (~10) of Protemnodon are known only from the Plio-Pleistocene, with body mass
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