You could hardly accuse giant sthenurine kangaroos of being fussy eaters.
Researchers have long suspected these now extinct “short-faced” kangaroos, with their large jaws and teeth and shortened muzzles, were able to consume bulky foods like mature leaves, stems and branches.
A new biomechanics study published in PLOS ONE today has lent further weight to this theory.
It shows the animals had skulls able to withstand the forces produced by the powerful bites they would have needed to employ to tackle their tough diets.
Sthenurine kangaroos — a completely separate lineage to modern-day roos — reached their peak during the Pliocene and Pleistocene epochs, when they numbered some 26 different species.
But the Pleistocene was also when Australia was going through the last Ice Age, and food would not have been plentiful.
The sthenurines’ diverse diets would have come in handy.
“Basically, it meant when times were tough they could certainly eat more things than what others could,” said zoologist D. Rex Mitchell, who carried out the research at the University of New England.
The biomechanics of a bite
Dr Mitchell’s study focused on one species in particular, Simosthenurus occidentalis, which lived on the Australian mainland and in Tasmania until about 42,000 years ago.
“This species I looked at was probably comparable in height to a modern-day kangaroo or an adult human, but they were about 30 kilograms heavier than kangaroos that are around today,” he said, or around 118kg on average.
Using CT scans of a S. occidentalis skull, Dr Mitchell was able to create three-dimensional models of the skull on which he could run biomechanical simulations.
He tested these simulations against koala models, as koala skulls were the most similar in shape.
It’s an interesting approach because Dr Mitchell has taken skeletal elements, which is all we have with fossils, and tried to reinterpret the muscular anatomy around that, said Natalie Warburton, a zoologist and palaeontologist from Murdoch University who was not involved in the study.
In S. occidentalis, the animal’s pre-molar and molar teeth, or its cheek teeth, were really close to the joints connecting its jawbone to its skull.
That’s surprising, Dr Mitchell said, because while this increases the mechanical efficiency of the animal’s bite, so more muscle force is turned into bite force, it also runs the risk of the jaw dislocating every time the animal bites down on something hard with these teeth.
But then he noticed the animal’s enlarged cheekbones. If these bones were able to support larger muscles, they could balance the jaw joint.
“I ran a whole bunch of simulations where I increased the size of that muscle, and sure enough it increased its ability to bite with the rear teeth,” he said.
Resist the twist
The other feature of its skull that sets S. occidentalis apart from modern-day marsupials is its really wide forehead, which looks like a bony diamond, Dr Mitchell said.
“Bone isn’t cheap. And animals don’t tend to have a huge amount of bone unless it’s necessary for some reason,” he said.
He’s suggested that this enlarged frontal area is to resist the twisting (or torsional) forces on the skull that occur when the animal bites on something on one side of its face.
“You get high torsional forces happening along an axis that runs from the biting tooth across the skull to the opposite jaw joint,” Dr Mitchell said.
“And these bones follow that exact arc where those forces would be.”
If there’s a larger area of bone for the forces to spread through, it puts the skull under a lot less stress.
“By having a much broader skull, what’s happening is they’re spreading the load, and thus dissipating the forces over a larger surface area or through a larger volume,” Dr Warburton said.
“So it’s going to have less destructive effects at any one particular point.”
While the koala model showed a similar distribution, its skull was under more stress, which makes sense, according to Dr Mitchell, because it isn’t eating really thick vegetation like researchers think the short-faced kangaroos were.
While Dr Mitchell has drawn similar conclusions to other researchers in the field, his sophisticated analysis is more robust than previous studies because there are numbers attached to it, said palaeontologist Gavin Prideaux of Flinders University who was one of the reviewers of the paper.
“But we’d never put two and two together with regard to the whole expansion of that frontal area being for bracing of the skull under torsion,” Professor Prideaux said.
It’s a new line of investigation into the enigmatic short-faced kangaroos that supports some of the ideas that other types of analysis have thrown up, said palaeontologist Steven Salisbury of the University of Queensland, who was not involved in the study.
“The thing with these kangaroos is they are so different to modern kangaroos we’re familiar with, that trying to understand where they might have fitted into a sort of bigger ecological picture has been challenging,” Dr Salisbury said.
This study shows they’ve got really a specialised feeding apparatus in terms of the shape of their skull and their massive jaw muscles that you don’t really see in other kangaroos, he said.
“That sort of gives you a picture of this unusual, really big kangaroo maybe stripping vegetation off bark and leaves and things with its hands, and shoving that into its mouth and sort of chewing on it there, rather than nibbling away like we’re used to seeing kangaroos doing with grasses.”
While there are marsupials that eat tough plant material today, there’s certainly nothing that gnaws through thick branches like we think the short-faced kangaroo did, Dr Warburton said.
“This is a really interesting example of a marsupial that was performing a behaviour – in this case, munching on really tough bits of fibrous plant material – that we don’t really have any modern analogues of,” she said.
In fact, when Dr Mitchell went looking for a living animal with similar skull anatomy he made an unexpected discovery.
“We’ve got an animal that we can look at now and compare its morphology, and the way it eats, and come up with more ideas that links this species to what I’ve looked at.”
Using the method Dr Mitchell has pioneered could help us work out what might have driven short-faced kangaroos to extinction, Professor Prideaux said.
“If we’re going to try and work out what it was that made these species susceptible… we need to actually understand much more about their biology and ecology than we currently do,” he said.
“And so methods that [Dr Mitchell] has pioneered here are going to allow us to make some good steps forward.”