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The large marsupials of the middle Miocene that grazed the planet some 16 million years ago did not resemble the hippity-hoppity kangaroos we think of today. Christine Janis and colleagues at Brown University presented this rendering of the large sthenurine kangaroo, a subfamily member of the marsupial family Macropodidae, in PLoS ONE on October 15.
Their portrayal of a now-extinct ancestor of the relatively dainty present-day roo illustrates how this “short-faced” marsupial’s unique body size and bone structure would make hopping on its two legs nearly impossible. Scientists speculated previously that the ancient sthenurine’s specialized forelimbs and a rigid lumbar spine would limit its ability to perform a pentapedal walk, or a low-speed gait that kangaroos commonly use instead of hopping.
The modern-day kangaroo propels itself forward by putting all four feet on the ground and using its tail as a sturdy fifth limb. After comparing bone and muscle measurements of the sthenurine with today’s kangaroos, Janis’s team found differences that would suggest this species had a completely different gait altogether.
The giant kangaroo, weighing an estimated 240 kilograms, appears to have held itself upright, lacking the specialized features that would allow it to hop rapidly. Its firm ankle joints and large hips and knees were likely strong enough to support its body weight on one leg at a time. Such features suggest that it walked like us, one plodding foot in front of the other, with a bit of a bowlegged gait. This stride may have enabled it to browse shrubs and trees for food without expending a lot of energy hopping from low to haute courses in a meal.
—Julia Calderone