A kangaroo unlike those we know

Turns out that Australia’s ancient kangaroo species have more in common with giant pandas that modern macropods.

That’s according to the University of New England’s D Rex Mitchell, author of a study published in the journal PLOS ONE about the  long-extinct Simosthenurus occidentalis.

Still hopping until about 42,000 years ago, S. occidentalis was a sthenurine kangaroo, sometimes called “short-faced” kangaroos – an entirely extinct group characterised in part by their heavily built skulls, large jaws and teeth, and shortened snouts.

The subfamily heavyweight was Procoptodon goliah, the largest macropod to have lived – a two-metre-tall, 200-kilogram-plus bruiser that most likely disappeared about 50,000 years ago. 

With an estimated average body mass of about 118 kilograms, S. occidentalis was several shoe sizes down on P. goliah, but still heftier than the largest living macropod, the red kangaroo (Macropus rufus), which tops out at about 90 kilograms.    

Past research has suggested that the robust skull features of sthenurines indicate that these animals had powerful jaws adapted for a diet of tough foods such as mature leaves, stems, and branches. 

Mitchell hypothesised that if this is true, the kangaroos’ skulls should also be built to resist the forces that such powerful bites would put on skull bones and joints.

He created a digital model of a S. occidentalis skull, ran a series of bite simulations, then compared them to the bite of the koala (Phascolarctos cinereus) – the extant animal considered to have the most similar ecology and skull type. 

Differences between the two include the positioning of S. occidentalis’s cheek teeth. They extend further back on the jaw bones than do a koala’s and would thus have put much more force on the jaw joint and increased the likelihood of jaw dislocation.

The simulations revealed that S. occidentalis could produce and withstand comparatively heavy forces when biting unilaterally – specifically, when feeding very tough materials into the cheek teeth on one side of its mouth, much as a giant panda does with a bamboo stem. 

S. occidentalis’s cheekbones, Mitchell found, supported large muscles that would prevent its jaw from being dislocated during strong biting, and that the bones of the front and roof of the skull formed an arch that would resist twisting forces during bites.

These attributes support the suggestion that sthenurine kangaroos’ skulls were well adapted to producing and withstanding powerful bite forces, which would have allowed them to eat tough, low-nutrition foods that may have been inaccessible to other species. 

Mitchell notes that S. occidentalis – an extinct species – appears adapted to exploit tough foods to a greater extent than any living Australian herbivore, meaning these kangaroos represent a feeding behaviour and ecology that’s no longer seen on the continent.

“The skull of the extinct kangaroo studied here differs from those of today’s kangaroos in many of the ways a giant panda’s skull differs from other bears,” says Mitchell. 

“So, it seems that the strange skull of this kangaroo was, in a functional sense, less like a modern-day kangaroo’s and more like a giant panda’s.”