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Living kangaroos, wallabies and rat-kangaroos (Macropodoidea) constitute the most ecologically diverse radiation of Australasian marsupials. Indeed, even their hallmark bipedal hopping gait has been variously modified for bounding, walking and climbing. However, the origins of this locomotory adaptability are uncertain because skeletons of the most ancient macropodoids are exceptionally rare. Some of the stratigraphically oldest fossils have been attributed to Balbaridae—a clade of potentially quadrupedal stem macropodoids that became extinct during the late Miocene. Here we undertake the first assessment of balbarid locomotion using two-dimensional geometric morphometrics and a correlative multivariate analysis of linear measurements. We selected the astragalus and pedal digit IV ungual as proxies for primary gait because these elements are preserved in the only articulated balbarid skeleton, as well as some unusual early Miocene balbarid-like remains that resemble the bones of modern tree-kangaroos. Our results show that these fossils manifest character states indicative of contrasting locomotory capabilities. Furthermore, predictive modelling reveals similarities with extant macropodoids that employ either bipedal saltation and/or climbing. We interpret this as evidence for archetypal gait versatility, which probably integrated higher-speed hopping with slower-speed quadrupedal progression and varying degrees of scansoriality as independent specializations for life in forest and woodland settings.

The exceptionally diverse macropodiform’s (kangaroos, rat-kangaroos and their fossil allies) currently have a fossil record that spans from the late Oligocene to the Holocene with an Australasian widespread fossil occurence. The origins of the macropodiforms are believed to have been during the Eocene possibly having split from the Phalangeridae. This is largely based on molecular data as there is a complete lack of macropodiform fossil material prior to the late Oligocene leaving the origins of the macropodiforms to be largely speculative. Thus, late Oligocene rat-kangaroo dental fossil elements associated with Palaeopotorous priscus (which shares characteristics observed in both phalangerid and macropodiforms) were examined to get a better insight into the potential origins of the macropodiforms. The results obtained suggested that P. priscus is currently the most basal macropodiform known. Furthermore, due to the absence of adequate macropodiform post-cranial material, the functional eco-morphological interpretation of various macropodiform fossil taxa has been based largely on cranial and dental characteristics. Consequently, the examination of Miocene Balbaridae kangaroo’s (Balbaroo nalima and an untaxonomised balbarid) post-cranial elements was undertaken, suggesting an array of locomotion types (similar to living macropodiforms) and a likely persistent arboreal and quadrupedal lifestyle. In addition, the Pleistocene Protemnodon anak’s post-cranial material are examined, proposing a probable combination of quadrupedal bounding and a slow walking gait alternative to the eminent fast bipedal saltation seen in almost all extant macropodiforms.