Modern kangaroos are a diverse bunch of species, but we know them all for their tendency to hop. According to new research published in PLOS ONE, their ancient relatives probably had a gait that would look pretty strange to us: They walked upright on two legs, like we do.

The now extinct sub-family of sthenurine kangaroos had legs made for walking, researchers now claim. Some species of the giant, short-faced creatures weighed over 500 pounds -- nearly three times the weight of their modern relatives. At that size, hopping would have become unwieldy.

"At best, they'd have been really clumsy hoppers," said study author Christine Janis, a professor of ecology and evolutionary biology at Brown University. But she and her colleagues suspected that sthenurine was more than just awkward.

Modern kangaroos hop for speed, but they use a specific kind of amble when they want to go slowly: They use their tail as a third leg.

A previous study found that the sthenurine couldn't have managed that movement. These kangaroos had rigid spines. They wouldn't have had springy, flexible tails ready to support their immense weight.

"They would have had to do something else to get around slowly," Janis said.

In examining the other components of sthenurine skeletons, Janis and her colleagues found other differences between these and modern kangaroos. The big-boned bruisers had larger hip and knee joints, as well as a broad, flared pelvis. This would allow for larger gluteal muscles than modern kangaroos possess, which would let them support their body weight on one leg at a time.

"All of the differences we found were suggestive of putting weight on one foot at a time," Janis said.

Janis and her colleagues believes that sthenurines initially used this walking gait for slow locomotion, but hopped at high speed just like other kangaroos. Over time, the differences in their gait allowed some of them to evolve to a huge size -- and those sthenurine had to give up hopping for good.

"Hopping is a tricky gait, and modern kangaroos are near the limit, in terms of size," Janis said. It could be that having walking to fall back on is what allowed sthenurines to get so big.

This is just one of many cases of humans believing that extinct animals must have moved and acted like their modern-day relatives, Janis said. In many cases, the truth is a lot more alien.

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