With a weird knobbly face and looking like a cross between a lizard and a hippo, this pre-reptile is now thought to be the earliest known animal to have walked upright on all fours. Know as Bunostegos akokanensis, it lived about 260-million-years ago, plodding around what is now the African country of Niger. Belonging to a group of animals called pareiasaurs, some argue that they eventually gave rise to turtles, though this is debated.

“A lot of the animals that lived around the [same] time had a similar upright or semi-upright hind limb posture, but what's interesting and special about Bunostegos is the forelimb,” explains Morgan Turner, who co-authored the paper published in The Journal of Vertebrate Paleontology. “The elements and features within the forelimb bones won't allow a sprawling posture. That is unique.”

It was previously thought that at the time Bunostegos was stomping around Pangea, the supercontinent that split up to form the continents we know today, all pareiasaurs moved with a “sprawling” gait. This is when the animal’s limbs stick out from the side of the body and then slant down to hit the floor, as seen in modern-day lizards and salamanders. It is the most primitive form of locomotion on land, occurring before animals evolved a fully erect stance.  

Now, it appears that transition happened earlier than previously thought. Originally discovered in 2003, the fossils of Bunostegos caught the eye of paleontologists at Brown University. When looking at the forearms of several individuals, and the joints between the bones, it became more and more apparent to Turner that the animals’ limbs must have been positioned directly below the body.   

^{The shoulder joint (1), humerus (2), knee-like elbow joint (3), and a longer lower arm (4) together make the case that Bunostegos stood with its legs under its body. Morgan Turner/Brown University} 

The shoulder joint is at such an angle that it would have been physically impossible for the humerus, or upper arm bone, to have stuck out sideways. In addition to this, while the humerus of most sprawling animals is twisted – to allow the lower arm and foot to reach the floor – this is not seen in the bones of Bunostegos, indicating that it was not positioned in the same way. And the evidence doesn’t end there.

The elbow joint in most sprawling pareiasaurs is incredibly flexible, but Turner did not find that to be the case in Bunostegos. Instead, the joint was a lot more similar to our knee joints, only allowing for a forward and backwards motion of the lower arm and foot. Finally, confirming the conclusion in Turner's opinion were the relative lengths of the upper and lower arm, with the latter being longer than the former, as is common in most non-sprawling animals.

“Posture, from sprawling to upright, is not black or white, but instead is a gradient of forms,” says Turner. “There are many complexities about the evolution of posture and locomotion we are working to better understand every day. The anatomy of Bunostegos is unexpected, illuminating, and tells us we still have much to learn.”

Because of this gradient, Turner expects there were probably other pareiasaurs living at the time with a similar gait to Bunostegos, but are yet to be found.