First Creature To Walk On All Fours Revealed By Pre-Reptile Fossils, Researchers Say

After closely examining the forelimbs of a pre-reptile fossil species known as Bunostegos akokanensis, Brown University researchers concluded that it is the oldest known creature to walk upright on all four legs. 

Bunostegos is a 260-million-year-old pre-reptile that roamed the supercontinent Pangea munching on plants. According to a news release, scientists previously thought that all Permian herbivores had a sprawling body type -- where their limbs would extend from the sides of their body and slant downward from their elbows, similar to some modern lizards. However, Bunostegos fossils, which were originally found in Niger, Africa, in 2003 and 2006, paint a different picture.

"A lot of the animals that lived around the time had a similar upright or semi-upright hind limb posture, but what's interesting and special about Bunostegos is the forelimb, in that it's anatomy is sprawling-precluding and seemingly directed underneath its body--unlike anything else at the time," Morgan Turner, lead author and graduate student at Brown University, said in the release. "The elements and features within the forelimb bones won't allow a sprawling posture. That is unique."

From their recent analysis, the researchers concluded that the Bunostegos resembled modern cows in both size and posture. However, unlike grass-grazing cows today, this pre-reptile was also suited with boney armor down his back and a knobby skull, according to Linda Tsuji, co-author from the Royal Ontario Museum.

In their study, the researchers explained how Bunostegos was able to stand tall. The answers lie in the pre-reptiles' shoulder joint, humerus, elbow and ulna. Its shoulder faced down so that the humerus, the bone running from the shoulder to the elbow, was directly underneath its body. This is different than sprawlers, where the humerus sticks out toward the side of the body. The pre-reptile's elbow also differed from sprawler's in that it was more like a human knee -- with a limited range of motion, capable of only swinging back and forth. In contrast, sprawlers were able to swing their forearms out to the side. Finally, the researchers noted that the Bunostegos' ulna is longer than the humerus, a common characteristic among non-sprawlers.

According to the release, the Bunostegos's posture suggests that it was an outlier. This makes sense based on the natural habitat it would have lived in 260 million years ago, where food sources would have been spread out. Being able to walk on all fours was necessary for the Bunostegos to travel long distances for food.

"Posture, from sprawling to upright, is not black or white, but instead is a gradient of forms," Turner explained in a statement. "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."

Their study was recently published in the Journal of Vertebrate Paleontology.

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