THEY had faces like rabbits and some were 2 metres tall. But now it seems that an extinct giant kangaroo didn't hop – it walked.
Known as "short-faced giant kangaroos", sthenurines roamed Australia for 12.5 million years before being wiped out 30,000 years ago. No one knows what killed off these relatives of modern kangaroos, although they may have struggled as Australia's climate grew more arid.
Unlike kangaroos today, these ancient giants walked just like us. "All our evidence fits with these animals leaning on one leg at a time, like humans," says Christine Janis of Brown University in Providence, Rhode Island.
She and her team took hundreds of measurements of the bones of 66 living species of the kangaroo family and 78 extinct species, including 71 from sthenurines, to calculate the likely size and function of each animal's bones and muscles.
They found that the ill-fated sthenurines had bone structures resembling those of animals that move by shifting weight from one foot to the other, like humans and apes (PLOS One, DOI: 10.1371/journal.pone.0109888). A flange at the base of their shin bones, similar to those found in horses and humans, would have prevented their feet from collapsing sideways under the weight of their body.
Like walking primates, their pelvises fanned out at the rear. "They had big bums, and much more room for these large gluteal muscles than today's kangaroos," says Janis. These muscles would have supported each leg during walking.
Sthenurines had stiffer spines and shorter tails than modern kangaroos, which use their tails like a fifth leg. "It's reasonable that the ancestors of modern-day kangaroos moved differently," says Max Donelan of Simon Fraser University in Burnaby, Canada. "If pentapedal walking did not make biomechanical sense to these large, extinct kangaroos, they would have likely found a different solution."
Janis suggests they were browsers, taking advantage of their height and bipedalism to browse trees and shrubs for berries or other food, without having to waste energy hopping between them.
This article appeared in print under the headline "Extinct giant kangaroos didn't hop – they walked"