An analysis of bones from a pre-historic pre-reptile, Bunostegos akokanensis,suggests that the creature is the first known to have ever stood upright on all fours. According to io9, the pareiasaur "lived during the Permian era some 260 million years ago."
Most pareiasaurs are believed to have had sprawling appendages—somewhat akin to modern-day lizards. But the one in question had legs entirely underneath its body.
One of the study's authors described the animal's appearance: "Imagine a cow-sized, plant-eating reptile with a knobby skull and bony armor down its back."
It's believed the animal was a bit isolated and wandered the arid deserts of the former Pangean supercontinent.
Scientists say that walking upright is a more energy-efficient form of posture than the sprawling position.
Thus, it's plausible that the area in which the pareiasaur lived contributed to its evolution of an upright way of life—if it had to say, walk longer to get its mealsdue to food scarcity.
The discovery rewrites the story of upright walking.