A researcher has revealed intriguing new details about a group of giant prehistoric kangaroos according to a study published in the open-access journal PLOS ONE.© N. Tamura Artistic reconstruction of the short-faced kangaroo, Simosthenurus occidentalis. According to the scientist, the skulls of these so-called “sthenurine” kangaroos were highly specialized, providing the animals with a powerful…
A researcher has revealed intriguing new details about a group of giant prehistoric kangaroos according to a study published in the open-access journal PLOS ONE.
© N. Tamura
Artistic reconstruction of the short-faced kangaroo, Simosthenurus occidentalis.
According to the scientist, the skulls of these so-called “sthenurine” kangaroos were highly specialized, providing the animals with a powerful bite which enabled them to eat tough foods.
Unlike their modern counterparts, these kangaroos likely walked on two legs, much like humans, rather than hopping around to move. This animal family—sometimes referred to as “short-faced kangaroos”—had all gone extinct by the end of the Late Pleistocene Period (between around 126,000 and 5,000 years ago.)
“Sthenurine kangaroos are an entirely extinct group of robust marsupial herbivores that arose during the Miocene Epoch [around 23 to 5.3 million years ago] and diversified prolifically during the Ice Age of Australia,” author of the study D. Rex Mitchell, from the University of Arkansas, told Newsweek.
“Currently, there are 26 species across 6 genera [groups of species] known from across Australia, including the largest kangaroo to have ever existed, Procopton goliah,” he said. “In general, they would have appeared quite different to modern-day kangaroos, with more thick-set bodies, long, muscular arms with extended fingers, often only a single big toe on each foot, and box-shaped heads that were shaped more like a koala’s.”
Sthenurine kangaroos featured heavily-built skulls, short snouts and large jaws and teeth, which scientists had previously suggested was an indication that they ate tough foods such as mature leaves, stems and branches.
To determine if this was indeed the case, Mitchell tested the idea that the construction of the skulls of these kangaroos should be sturdy enough to deal with the powerful forces which would be generated by their bites.
To do this, Mitchell digitally recreated the skull of one sthenurine kangaroo species known as Simosthenurus occidentalis which roamed what is now Australia in the Late Pleistocene until going extinct around 42,000 years ago.
“A study I published back in January examined this species using shape analysis and biomechanical simulations,” Mitchell said. “The results showed that the skull of the extinct kangaroo was, out of a range of modern-day marsupial herbivores tested, shaped most similar to a koala.”
However, this kangaroo also differed from all species it was compared against in having a deeper skull, a very wide forehead, and enormous cheekbones. The latest study’s aim was to identify the significance of these reinforced, bony features of the skull and whether they permitted specialized feeding behaviors.
“Simosthenurus occidentalis is a well-represented species of sthenurine, with remains found across much of the southern half of Australia. The species grew quite large, able to stand around as tall as an adult person and estimated to be around 118 kilograms [260 pounds]—around 30 kilograms heavier than the largest kangaroos of today. This makes it an interesting species to study, as a representative of Australia’s extinct megafauna,” he said.
Using the digital model, which was generated from CT scans of the skull of one S. occidentalis specimen, Mitchell simulated the animal’s bite and compared it to that of koalas.
“Simulations of biting at the molars and premolars were carried out to examine the distribution of forces across the skull,” he said. “Since the koala is the modern-day species with the most similar skull shape, the results were compared against models of a koala that underwent the same simulations.”
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These simulations indicated that the cheekbones of S. occidentalis supported large muscles that prevented dislocation of the jaw when the animal used its powerful bite.
“Interestingly, the short-faced kangaroo models were found to have a much higher risk of injury than the koala models during biting at the back teeth,” Mitchell said. “This is because its teeth were much larger and extended farther back towards the jaw joints. This greatly increased how hard the animal could bite, but also increased the risk of jaw dislocation when biting.”
“However, I found that an enlargement of a muscle located on the inner surface of its immense cheekbones would help to reduce this risk,” he said. This muscle is also enlarged in the giant panda, another similarly sized animal that feeds on thick, resistant vegetation such as bamboo.
Furthermore, the scientist found that the bones of the front and roof of the skull provided sufficient structural support to resist the twisting forces that would have been generated during these bites.
“This study revealed that the short-faced kangaroo model could withstand torsion much more effectively than the koala during such bites,” Mitchell said. “Its deep skull and broad forehead could spread stress more evenly, preventing excessive twisting and protecting it from damage.”
This finding supports previous suggestions about short-faced kangaroos that the toughest, thickest vegetation that it could have eaten, such as the woody twigs and branches of trees and shrubs, may have been fed directly to its premolars and molars to be crushed or otherwise broken apart. Such actions would appear similar to how giant pandas crush bamboo,” he said.
Taken together, these results led Mitchell to the conclusion that kangaroos in the sthenurine group were able to produce powerful bites due to these adaptations, enabling them to eat tough foods that other species were not able to consume.
“The research provides support for the hypothesis that at least some short-faced kangaroos could persist on thick and woody, poor-quality vegetation at times of low productivity, such as during droughts, and prolonged glacial periods and associated continental drying,” he said. “The ability to consume parts of plants that other herbivores of the time could not would have offered them a competitive edge when times were tough.”
In this sense, S. occidentalis is similar to modern animals such as the giant panda, which also eats tough food in the form of bamboo.
“The similarities between the skulls of this kangaroo and the giant panda offer a good example of convergent evolution, whereby similar features of the skull have independently evolved to serve potentially similar functions,” Mitchell said.
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