Unlike most other creatures of its era, which were sprawlers that had legs jutting out of the side of their bodies, a pre-reptile known as Bunostegos akokanensis stood upright and walked on all four limbs like a hippo or cow, according to a new Journal of Vertebrate Paleontology study.
This discovery marks the first time that upright posture has been found in a creature this old, said lead author Morgan Turner, a graduate student at Brown University who analyzed the remains of the creature while working at the University of Washington’s Burke Museum of Natural History and Culture.
Turner told redOrbit that her team was “very surprised” to find that Bunostegos walked on all fours. “All the four-legged land animals that lived at the time have been shown to be sprawlers, with the legs coming out of the side of the body,” she said. “We don’t see upright posture, with the legs underneath the body, in both the forelimb and the hind limb in a single animal until much later, in mammals and in dinosaurs.”
Bunostegos would have lived 260 million years ago on the supercontinent Pangea. The pre-reptile was “a large, herbivorous, cow-sized animal” that had “boney armor running down along its back,” and it (along with its closest relatives) were some of “the first large-bodied animals on land.”
Finding could alter understanding of posture evolution
Turner and her colleagues looked at the skeletons of several individual specimens of Bunostegos, and made four observations that allowed them to conclude that the creature walked upright, with its legs entirely beneath its body. The shoulder joint, humerus, knee-like elbow hinge and longer ulna combined to convince the researchers that the pre-reptile was not a sprawler.
The shoulder joint faced down so that the humerus (the bone running from shoulder to elbow) would be vertically oriented underneath, preventing this bone from sticking out to the side, they explained. The humerus is not twisted like those in sprawlers, suggesting that the food would only be able to touch the ground if its elbow and shoulders aligned beneath the body.
In addition, the Bunostegos had a more limited elbow joint compared to sprawling pareiasaurs, which tend to have greater mobility at the elbow. Its forearms bones combined with the humerus to form a hinge-like joint that limited the forearm to a back-and-forth movement like the human knee. Finally, the ulna is longer than the humerus – a trait which is common in non-sprawlers.
“Bunostegos is much further back on the evolutionary tree than anything else that exhibits this posture,” Turner told redOrbit. “This new finding is surprising and perhaps hints at a larger story about posture and locomotion evolution. Posture, from sprawling to upright, is not black or white, but instead is a gradient of forms. There are many complexities about the evolution of posture and locomotion many scientists are working to better understand every day. The anatomy of Bunostegos is unexpected, illuminating, and tells us we still have much to learn.”
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Feature Image: Megan Turner
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