A method, based on femoral circumference, allowed us to develop body mass estimates for 11 extinct Pleistocene megafaunal species of macropodids (Protemnodon anak, P. brehus, P. hopei, P. roechus, Procoptodon goliah, ‘P.’ gilli, Simosthenurus maddocki, S. occidentalis, Sthenurus andersoni, S. stirlingi and S. tindalei) and three fossil populations of the extant eastern grey kangaroo (Macropus giganteus). With the possible exception of P. goliah, the extinct taxa were browsers, among which sympatric, congeneric species sort into size classes separated by body mass increments of 20–75%. None show evidence of size variation through time, and only the smallest (‘P.’ gilli) exhibits evidence suggestive of marked sexual dimorphism. The largest surviving macropodids (five species of Macropus) are grazers which, although sympatric, do not differ greatly in body mass today, but at least one species (M. giganteus) fluctuated markedly in body size over the course of the Pleistocene. Sexual dimorphism in these species is marked, and may have varied through time. There is some mass overlap between the extinct and surviving macropodid taxa. With a mean estimated body mass of 232 kg, Procoptodon goliah was the largest hopping mammal ever to exist.

Using demonstrated relationships between body mass and humeral and femoral circumferences, we calculate the weight of the only specimen of Thylacoleo carnifex known from a near-complete skeleton. Body weights of 112–143 kg were estimated for this individual, from Moree, north-western New South Wales. Extrapolating on the basis of geometric similtude, we further estimated the weight of the largest T. carnifex for which we had cranial data at 128–164 kg. Moreover, estimates for at least three of the thirteen available specimens exceeded 124–160 kg, suggesting that individuals of this size were common. Our estimates of average weight for the species range from 101 to 130 kg. These results clearly show that Pleistocene Australia had a 'large' cat equivalent and that 'large' terrestrial predator niches were not then occupied exclusively by reptiles.They may also diminish the argument that soil-nutrient deficiency constrained the evolution of large mammalian carnivores on this continent in the Pleistocene. Similarly, we posit that prima facie evidence for reptilian domination of terrestrial carnivore niches during the Miocene is wanting, although it is conceded that far more detailed investigation is required to comprehensively test these hypotheses. Earlier studies have drawn parallels between T. carnifex and sabre-toothed predators, thought to have specialised in hunting particularly large and powerful prey. Taken in the context of upwardly revised weight estimates, we argue that Pleistocene marsupial lions may have dispatched even Diprotodon-sized animals. But again, more comprehensive study, including thorough biomechanical design analysis of the post-cranial skeleton in particular, will be required to thoroughly illuminate the predatory habitus and general ecology of Australia's largest and most specialised marsupial carnivore.

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