Proximal Humerus Morphology Indicates Divergent Patterns of Locomotion in Extinct Giant Kangaroos
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ORIGINAL PAPER
Proximal Humerus Morphology Indicates Divergent Patterns of Locomotion in Extinct Giant Kangaroos Christine M. Janis 1,2
&
James G. Napoli 3
&
Coral Billingham 1 & Alberto Martín-Serra 4
# The Author(s) 2020
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 weightbearing 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. Keywords Macropodoidea . Macropodidae . Sthenurinae . Protemnodon . Functional anatomy . Humerus . Locomotion
Introduction The familiar locomotor mode of kangaroos is hopping: that is, bipedal locomotion using the hind limbs in tandem, unaided by the forelimbs. But, 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). In the Pleistocene there was a diversity of kangaroo Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10914-019-09494-5) contains supplementary material, which is available to authorized users. * Christine M. Janis [email protected] 1
Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK
2
Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912, USA
3
Richard Gilder Graduate School, Division of Paleontology, American Museum of Natural History, New York, NY 10024, USA
4
Departamento de Ecología y Geología. Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain
species much larger than the extant ones, the so-called “giant kangaroos.” These include the giant short-faced browsing kangaroos (subfamily Sthenurinae; family Macropodidae), and the so-called “giant wallaby,” Protemnodon (subfamily Macropodinae; family Macropodidae). Here, we consider the mode of slow locomotion in these large extinct kangaroos by an examination of the morphology of their proximal humeri in comparison with exta
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