Evolution of Hind Limb Proportions in Kangaroos (Marsupialia: Macropodoidea)

Kangaroos (Macropodoidea: Marsupialia) are a characteristic group of Australo-New Guinean mammals that diversified during the geographic isolation of the Australian continent in the Cenozoic. They are first recorded in the Late Oligocene, although the clade diverged from other diprotodontians around 38 million years ago (mya; Westerman et al., 2002), with early forms perhaps resembling small arboreal ‘phalangerids’ (Flannery, 1982). Living macropodoids vary widely in body size ( 60 kg in larger species of Macropus), and show a high degree of ecological diversity. They include forms specialized for climbing (e.g., Dendrolagus), burrowing (e.g., Bettongia leseur), and occupation of closed rainforest/woodland (e.g., Hypsiprymnodon, Setonix) through to open temperate/tropical and/or arid zone grassland (e.g., Macropus). Despite this variability, the appendicular skeleton of macropodoids is remarkably conservative with all members of the group showing similar modifications (particularly in the long bones of the hind limb, tarsus, and pes) favoring a bipedal hopping gait. Windsor and Dagg (1971) standardized terminology for kangaroo locomotion designating ‘slow pentapedal progression’ as that involving synchronous use of the limbs and tail (present in all macropodoids and extensively used by species of Dorcopsis; Bourke, 1989), ‘walking’ as a gait involving asynchronous use of all limbs (confined to species of Dendrolagus; Windsor and Dagg, 1971), ‘quadrupedal bounding’ as movement employing synchronous use of all limbs (present in species of Dendrolagus, Windsor and Dagg, 1971; Flannery et al., 1996; and H. moschatus, Johnson and Strahan, 1982), and bipedal hopping characterized by synchronous use of the hind limbs only (used at high speeds by 2. Evolution of Hind Limb Proportions in Kangaroos (Marsupialia: Macropodoidea)