3D scan shows first soft tissue outline in Diprotodon-like ‘wanderer’ fossil

More than 3.5 million years ago, a hog-sized marsupial could be found walking the edge of what is now the Simpson Desert. 

Now, studies of the new Ambulator keanei’s bonesunearthed by a team at Flinders University in 2017 – show the diprotodontid adapted a heeled walk perfect for moving all over the plains of ancient Australia.

“They’ve essentially modified their hand into the secondary foot,” Flinders University palaeontologist Jacob van Zoelen told Cosmos.

“And they’ve done this in a way that we’ve never really seen in any other animal.”

The discovery published in Royal Society Open Science, was supported by a 3D scan of Ambulator’s whole foot before the bones were removed from the rock concretion around it.

“We CT scan this foot and actually we’ve found preservation of soft tissue in inside the concretion,” says van Zoelen.

“Your foot has these inner and outer layers of fat capsules which act kind of like shock absorbers for your feet. We’ve found the outline of these capsules on the outside of the foot of this animal.

“That was an amazing find.”

Jacob van zoelen and the skeleton
Jacob Van Zoelen with the partial skeleton of Ambulator keanei. Credit: Flinder University

In fact, this is the first diprotodontid specimen discovered with associated soft tissue structures.

There are many types of diprotodontid that roamed Australia between 50,000 and 26 million years ago; they are also the largest documented marsupials and occupied a wide variety of environments.

“They are an incredibly diverse group,” says van Zoelen.

“They occupy all these different ecosystems and habitats. Diprotodon is found all over – it’s quite generalist – but you get others which are living in forest environment or mountain ones.”

This new genus was thought to travel longer distances in open habitats to gain more resources for survival.

While Ambulator’s ‘boots’ are made for walking, the way they do so is different to many large modern animals such as elephants or rhinos. These animals all have adapted their feet to walk on their toes, but Ambulator would have walked on its heels instead.

They walk on the palms and the heels,their digits are greatly reduced,” says van Zoelen.

“They walk on the palms and the heels of their feet, and their digits are greatly reduced and have lost a lot of flexion [ability to bend] in their fingers and toes,” says van Zoelen.

The researchers was able to observe and check this using ‘trackways’ – ancient fossil footprints still visible today.

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Hand and foot impression of Diproton optatum. Notice the absence of digit impressions. Photo by AB Camens (Flinders University)

Regular rain has meant the Tirari Formation where the Ambulator fossil was found has been unreachable for several years.

Returning to the site is particularly important for the Flinders team, as the soft tissue specimen was damaged after the 3D scan (available online) was done. The scan remains the only evidence of this tissue.

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