New US research has shed light on an extinct kangaroo that was three times the size of the largest existing kangaroos.
A giant extinct kangaroo that lived in the Australian outback 100,000 years ago was so heavy it would have walked rather than hopped, research has shown.
Scientists said the 2.7-metre-tall creature, which had a round rabbit-like face, would have been an unnerving sight as it strode around on two legs.
The "short-faced" sthenurine weighed as much as 250kg and was three times the size of the largest present-day kangaroos.
Scientists who conducted a detailed study of its anatomy believe the marsupial was just too heavy to hop.
"I don't think they could have gotten that large unless they were walking," said lead researcher Professor Christine Janis, from Brown University in Providence, US.
The team analysed bones from more than 140 past and present kangaroo and wallaby skeletons.
Unlike large modern red and grey kangaroos, sthenurines lacked specialised features geared for rapid hopping.
Yet their bone structure indicated they had an upright posture and were able to support their weight on one foot at a time.
The creatures had large hips and knees and stable ankle joints that would have helped them walk on two legs.
Their hands were poorly suited for moving on all fours, but adapted for foraging.
"People often interpret the behaviour of extinct animals as resembling that of the ones known today, but how would we interpret a giraffe or an elephant known only from the fossil record?" said Prof Janis.
"We need to consider that extinct animals may have been doing something different from any of the living forms, and the bony anatomy provides great clues."
The research is reported in the online journal Public Library of Science ONE.
Whether or not reliance on walking rather than faster and more efficient hopping led to sthenurines becoming extinct about 30,000 years ago is unknown.
Prof Janis said they may have struggled to elude human hunters, or been unable to migrate far enough to find food as the climate became more arid.