"Exploring the Long-Distance Walker: Unveiling Ambulator keanei, a 3.5 Million Year Old Marsupial from Australia"
Key Highlights :
The Australian continent has been home to a variety of animals, including the diprotodontid marsupials which have been extinct for the past 40,000 years. Recently, Flinders University paleontologists have used advanced 3D scans and other technology to take a new look at the partial remains of a 3.5 million year old marsupial from central Australia, and have named a new genus of diprotodontid Ambulator, meaning walker or wanderer.
The Ambulator keanei, found on the Australian Wildlife Conservancy's Kalamurina Station in northern South Australia by Flinders University researchers in 2017, belongs to a species in the family Diprotodontidae, a group of four-legged herbivores that were the largest marsupials to ever exist. The largest species, Diprotodon optatum, weighed up to 2.7 tons.
The researchers believe that the locomotory adaptations of the legs and feet of this quarter-ton animal would have made it well suited to roam long distances in search of food and water when compared to earlier relatives. This is because during the period when Ambulator keanei was alive (the Pliocene), there was an increase in grasslands and open habitat as Australia became drier.
Jacob van Zoelen, a Ph.D. candidate at the Flinders University Palaeontology Laboratory, explains that diprotodontids were distantly related to wombats and kangaroos are to possums, so unfortunately there is nothing quite like them today. As a result, paleontologists have had a hard time reconstructing their biology.
Van Zoelen further explains that most large herbivores today such as elephants and rhinoceroses are digitigrade, meaning they walk on the tips of their toes with their heel not touching the ground. However, diprotodontids are plantigrade, meaning their heel-bone (calcaneus) contacts the ground when they walk, similar to what humans do. This stance helps distribute weight when walking but uses more energy for other activities such as running.
The researchers also found that diprotodontids display extreme plantigrady in their hands as well, by modifying a bone of the wrist, the pisiform, into a secondary heel. This 'heeled hand' made early reconstructions of these animals look bizarre and awkward. Van Zoelen says that 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.
The researchers believe that the Ambulator keanei’s locomotory adaptations would have allowed it to traverse great distances more efficiently, and eventually led to the evolution of the giant and relatively well-known Diprotodon.
Using 3D-scanning technology, the Flinders team was able to compare the partial skeleton with other diprotodontid material from collections all over the world. Encasing the foot of the individual was a hard concretion that formed shortly after death. By CT scanning the specimen, soft tissue impressions preserving the outline of the footpad were revealed.
The findings of this study were published in the journal Royal Society Open Science, and provide a better understanding of the locomotory adaptations of the Ambulator keanei. The researchers hope that this new information on the Ambulator keanei will help to further our understanding of the evolution of the diprotodontid marsupials.
