Australians are well-known for their general outlook that “roos” are no more than pests that do no small amount of crop damage and look like hopping rats. That disdain would likely be turn to the fear of God if they found themselves confronted by a pack of eight-foot-tall, quarter-ton, kangaroos known as sthenurines, a distant cousin of both gray and red kangaroos. Thankfully for Australians and Australia’s tourism industry this species went extinct around 30,000 years ago.

Monster Kangaroos Likely Walked, No Hopping

In a new paper recently published in the journal PLoS ONE, Brown University biologist Christine Janis postulates that they didn’t hop around like their present-day cartoonish cousins, but rather they just walked. She began her study several years ago along with two co-authors asking the simple question, “how they could hop at that size?” It turns out that after studying over 140 skeletons on sthenurines, they probably couldn’t hop at all as “the smallest of them were as big as the largest modern kangaroos.”

Monster Kangaroos: It just doesn’t add up

Her theory that they just poked without their trademark “hoppy” gait is fairly evident in a number of aspects including their teeth. The scientists found their teeth were more appropriate for eating leaves rather than feasting on grass like modern kangaroos. This led Janis and her colleagues to believe they could stand upright on two legs.

“They also had flared hipbones,” says Janis, with ample room for large gluteal muscles that would have permitted them to put weight on one leg at a time, something today’s kangaroos never do. Modern kangaroos amble around on all fours—or fives, if you count the tail, which they use for balance—when they’re browsing. When they want to go fast, they hop.

That’s possible only because they have flexible backs and stiff, substantial tails, which sthenurines lacked. The sthenurine hands, moreover, were unsuitable for bearing their weight. “They would have had trouble walking on all fours,” says Janis.

“Some have argued that the sthenurines might have had thicker tendons to compensate,” Janis says, “but that would have made the tendons less elastic. It just seems biomechanically unlikely.” Any arguments about tendons and other soft tissues are somewhat speculative in ancient specimens, of course. “Imagine that we only knew elephants as fossils,” says Janis. “How would we know for sure they had trunks?”

Whether hopping at me, which this paper essentially disproves, or walking at me, I’m running the opposite direction if ever confronted by an eight-foot tall kangaroo.