Australia's 550-pound marsupial walked with a 'heeled footpad' 3.5 million years ago

Large animals, like elephants and rhinoceroses, are digitigrades, as they walk on the tips of their toes and do not touch the ground with their heels. An Ambulator, on the other hand, was most likely a plantigrade with its heels entirely on the ground to distribute weight while walking, similar to humans. 

These locomotory adaptations would have allowed the creature to travel long distances back then. As per the authors, these footpad adaptations occurred at a time when Australia boasted dry conditions with more grasslands and open habitats. This likely made the Ambulator travel long distances for food and water. 

They also compared the remains to the diprotodontid family's earlier relatives, which included some of the largest four-legged herbivores marsupials. It was found that the species belongs to a new genus of diprotodontid Ambulator (walker or wanderer).

"Development of the wrist and ankle for weight-bearing meant that the digits became essentially functionless and likely did not make contact with the ground while walking. This may be why no finger or toe impressions are observed in the trackways of diprotodontids,” said Jacob van Zoelen, a Ph.D. candidate at the Flinders University Palaeontology Laboratory, in a statement.

In this family, the largest was Diprotodon optatum, which grew to be the size of a car and weighed 2.7 tons. Diprotodontids were an essential part of Australian ecosystems until about 40,000 years ago when the last species went extinct.

The paper is published in the journal Royal Society Open Science.

Study Abstract:

Diprotodontids were the largest marsupials to exist and an integral part of Australian terrestrial ecosystems until the last members of the group became extinct approximately 40 000 years ago. Despite the frequency with which diprotodontid remains are encountered, key aspects of their morphology, systematics, ecology and evolutionary history remain poorly understood. Here we describe new skeletal remains of the Pliocene taxon Zygomaturus keanei from northern South Australia. This is only the third partial skeleton of a late Cenozoic diprotodontid described in the last century, and the first displaying soft tissue structures associated with footpad impressions. Whereas it is difficult to distinguish Z. keanei and the type species of the genus, Z. trilobus, on dental grounds, the marked cranial and postcranial differences suggest that Z. keanei warrants genus-level distinction. Accordingly, we place it in the monotypic Ambulator gen. nov. We, also recognize the late Miocene Z. gilli as a nomen dubium. Features of the forelimb, manus and pes reveal that Ambulator keanei was more graviportal with greater adaptation to quadrupedal walking than earlier diprotodontids. These adaptations may have been driven by a need to travel longer distances to obtain resources as open habitats expanded in the late Pliocene of inland Australia.