Behold Bunostegos akokanensis, an ancient “pre-reptile” that lived during the Permian era some 260 million years ago. Researchers studying its fossilized remains have concluded that it stood upright on all fours, making it the earliest known creature to do so.
Pareiasaurs, an ancient group of parareptiles to which Bunostegos belonged, were large herbivores that flourished during the Permian period. They tended to be stocky, featuring short tails, small heads, robust limbs, and broad feet. And in virtually every instance, they were sprawlers—animals whose limbs extend outwards from the side of the body and then continue out or slant down from the elbow. Today, modern lizards offer a good example. But with the discovery of the cow-sized Bunostegos, paleontologists have learned that at least some pareiasaurs evolved the capacity to stand upright on all fours.
Computer rendering of Bunostegos (Credit: Marc Boulay).
The remains of Bunostegos were discovered in Niger by paleontologist and study co-author Linda Tsuji of the Royal Ontario Museum in 2003 and 2006. At first, the scientists figured it would be a sprawler like all other known pareiasaurs, but closer examination of the bones by a team that included Morgan Turner and Christian Sidor of Brown University told a different story. The results of their work now appears in the latest edition of the Journal of Vertebrate Paleontology.
“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,” noted Turner in a release. “The elements and features within the forelimb bones won’t allow a sprawling posture. That is unique.”
The shoulder joint (1), humerus (2), knee-like elbow hinge (3), and a longer ulna (4) together make the case that Bunostegos stood with its legs under its body. (Caption and image credit: Brown University/M. L. Turner et al., 2015.)
In the study, the researchers point to four distinct features indicative of an upright quadrupedal animal. From the Brown University 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.
What’s more, the environment in which Bunostegos lived may also explain why this creature evolved such a distinctive ability. It was an isolated pareiasaur that lived in the arid interior desert of the supercontinent Pangea. Scientists say that quadrupedal upright walking is a more energy-efficient posture than sprawling—a trait that’s important for animals having to make long journeys between meals. These paleontologists have shown that this physical characteristic dates very far back in time (a quarter of a billion years!), pushing back our conceptions as to when it first emerged.
Looking at Bunostegos, however, it’s fairly obvious that it was not the first animal to evolve this ability. It’s very likely that earlier animals, and possibly contemporaries, evolved this feature as well.
“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.”
Read the entire study at Journal of Vertebrate Paleontology: “The vertebrate fauna of the upper Permian of Niger—IX. The appendicular skeleton of Bunostegos akokanensis (Parareptilia: Pareiasauria).”
[ Brown University ]