The Bunostegos akokanensis was a cow-sized, plant-eating pre-reptile that roamed an extremely dry region of Pangea about 260 million years ago. But, it’s how the creature stands that has the paleontology world buzzing. The pre-reptile likely stood upright on all fours and is the earliest known creature to do so.
“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 its anatomy is sprawling — precluding and seemingly directed underneath its body — unlike anything else at the time,” said Turner. “The elements and features within the forelimb bones won’t allow a sprawling posture. That is unique.”
Bunostegos belonged to the a group of parareptiles known as Pareiasaurs. This family of animals were large herbivores that tended to be stocky, with short tails, small heads, robust limbs and broad feet. And besides Bunostegos, they were sprawlers – animals whose legs extended outward from the side of the body and then continue out or slant downwards from the elbow. Think of today’s lizards.
Bunostegos was unique in its posture and habitat. The remains of this Bunostegos were discovered in Niger by co-author Linda Tsuji of the Royal Ontario Museum in 2003 and 2006. 260 million years ago, Niger was extremely dry (just as it is today). Plants and water sources were probably few and far between. And it’s these distances between food and water sources that scientists believe was responsible for Bunostegos evolving the upright posture.
Scientists associate walking upright on all fours with a more energy-efficient posture compared to sprawling. Turner believes this posture may well have been vital for Bunostegos to survive.
Examining the skeleton
Image credit: Morgan Turner
The researchers highlight four features that support their findings.
From the release:
The shoulder joint — the glenoid fossa — is facing down such that the humerus (the bone running from shoulder to elbow) would be vertically oriented underneath. It would restrict the humerus from sticking out to the side.
Meanwhile Bunostegos’ humerus is not twisted like those of sprawlers. In a sprawler, the twist is what could allow the humerus to jut out to the side at the shoulder and then orient the forearm downward from the elbow. But the humerus of Bunostegos has no twist, suggesting that the foot could actually reach the ground only if the elbow and shoulders were aligned under the body, Turner said.
The elbow joint is also telling. Unlike in sprawling pareiasaurs, which had considerable mobility at the elbow, the movement of Bunostegos’s elbow is more limited. The way the radius and ulna (forearm bones) join with the humerus forms a hinge-like joint, and wouldn’t allow for the forearm to swing out to the sides. Instead, it would only swing in a back and forth direction like a human knee.
Finally, the ulna is longer than the humerus in Bunostegos, which is a common trait among non-sprawlers. “Many other sprawling four-legged animals have the reverse ratio,” Turner said.
Bunostegos walking upright is a significant finding. It shifts the time when paleontologists believed posture shows up in evolution even further back. And Turner wouldn’t be surprised if even earlier animals evolved to walk upright.
“Posture, from sprawling to upright, is not black or white, but instead is a gradient of forms,” Turner said. “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.”