Massive, prehistoric kangaroos didn't hop. Instead, researchers studying the locomotion of these extinct, rabbit-faced giants say they were built for bipedal walking -- one foot at a time, just like us. The findings were published in PLoS One this week. 

Sthenurine (“short-faced”) kangaroos are an extinct subfamily of roos, and they roamed the outback until about 30,000 years ago. They first appeared during the mid-Miocene around 12 million years ago, and later diversified into robust, large-bodied forms by the Pleistocene. The biggest of them all, Procoptodon goliah, grew to two meters high and weighed 240 kilograms -- that’s three times the size of the largest living roo. Various aspects of their anatomy, such as their rigid lumbar spine and slender arms, suggests a limited ability to perform the characteristic pentapedal (“five-legged”) walking we’ve come to associate with kangaroos with their muscular tails. And their teeth were better suited for browsing trees and shrubs for berries and other food, rather than grazing like today’s roos. 

To see if kangaroos of epic proportions were biomechanically capable of hopping locomotion, a trio of researchers led by Christine Janis of Brown compared nearly 100 measurements made on each of 144 kangaroo and wallaby skeletons: 66 modern animals spanning 45 species and 78 extinct individuals from 18 different genera. 

For example, here are the skeletons of the extinct Sthenurus stirlingi (top) and the modern-day eastern gray kangaroo, Macropus giganteus (bottom). 

They discovered several elements of sthenurine skeleton design that would have made them poor hoppers. First, hopping at fast speeds like today’s roos requires a flexible backbone, sturdy tail, and hands that can support their body weight. Sthenurines don’t appear to have had any of those.

Additionally, the lower end of their tibias have a flange that wraps over the back of the ankle joint to provide extra stability and support for each ankle. Living kangaroos, who almost always distribute their weight over both feet equally, don’t have that flange. Sthenurines also had bigger knee joints and a broad, flared pelvis. “They had big bums, and much more room for these large gluteal muscles than today's kangaroos," Janis tells New Scientist. 

Taken together, these features suggest that sthenurines put their weight on one leg at a time -- essential for walking on two feet. As Janis explains to Time: “just about everything we looked at made us go, ‘oh, that fits in.'” The bipedal striding gait enabled larger Pleistocene forms to evolve to body sizes where hopping was no longer a feasible form of rapid locomotion. “I don’t think they could have gotten that large unless they were walking,” Janis says in a news release. Tree kangaroos living in Papua New Guinea today occasionally walk like this.

What’s more, while these giants were proportionally more big-boned than their slender-boned relatives today, these modern-day roos are the “weird” ones, if you ask Janis. They’re proportionally very lightly built for their size, like cheetahs among cats. 

Images: Brian Regal (top) & Lorraine Meeker, American Museum of Natural History, modified from Wells and Tedford, 1995 (middle) via 2014 Janis et al., PLoS One