Myth of the QANTAS leap: perspectives on the evolution of kangaroo locomotion
The distinctive QANTAS ‘flying kangaroo’ motif of Australia’s national airline signifies what many people regard as the pinnacle of kangaroo evolution—a large-bodied marsupial specialized for endurance-hopping. However, while almost all extant macropodoids (the crown group including kangaroos) use hopping gaits to some extent, the fossil record reveals that the locomotory capabilities of extinct macropodoids were comparatively diverse. The earliest recognized Oligocene–middle Miocene macropodoids probably employed quadrupedal bounding, climbing and slower speed hopping as their primary modes of locomotion. Yet, all were apparently small-bodied (<12 kg), with larger-bodied (>20 kg) forms not appearing until the late Miocene coincident with increasing aridity and the spread of openly vegetated habitats. Hopping is functionally problematic at larger body sizes. Consequently, the later radiation of macropodids (kangaroos, wallabies and their relatives) achieved an optimal mass for efficient higher-speed hopping at ∼35 kg, with a theorized extreme limit of ∼140–160 kg. Modern kangaroos otherwise approach the peak mass range for such gaits at ∼50–90 kg, with the gigantic Pliocene–Pleistocene species of Protemnodon (‘giant wallabies’) at ∼100–160 kg likely being predominantly quadrupedal, and sthenurines (short-faced kangaroos) at ∼50–250 kg seemingly using bipedal striding. Here, we review the fossil evidence of macropodoid locomotion over the last ∼25 million years, and present preliminary analyses of limb bone and tarsal metric data. These indicate that the higher-speed endurance-hopping typical of modern large-bodied kangaroos was probably rare or absent in all but a few crown macropodoid lineages. The intrinsic gait variability of macropodoids has therefore diminished with Late Pleistocene megafaunal extinctions. As a result, the famous QANTAS ‘flying kangaroo’ actually depicts only one of what was once many successful locomotory strategies employed by macropodoids to conquer a range of terrestrial and arboreal habitats.
Christine M. Janis [email@example.com], Bristol Palaeobiology Group, School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK; Adrian M. O’Driscoll [firstname.lastname@example.org], Centre for Anatomical and Human Studies, Hull York Medical School, University of York, York YO1O 5DD, UK; Benjamin P. Kear [email@example.com], The Museum of Evolution, Uppsala University, Norbyvägen 16, Uppsala SE-75236, Sweden.