A cow-sized pre-reptile that lived 260 million years ago likely stood upright on all-fours, making it the earliest known creature to do so, researchers say.
To date all of the known pareiasaurs who roved the supercontinent of Pangea in the Permian era a quarter of a billion years ago were sprawlers whose limbs would jut out from the side of the body and then continue out or slant down from the elbow (like some modern lizards).
Morgan Turner, lead author of the new study, expected the pre-reptile Bunostegos akokanensis to be a sprawler too, but the bones of the animal's forelimbs told a different story.
"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," said Turner.
"The elements and features within the forelimb bones won't allow a sprawling posture. That is unique," said Turner, who performed the analysis under supervision of Professor Christian Sidor while he was a student at the University of Washington.
Turner, now a graduate student at Brown University, Sidor and co-authors characterised how Bunostegos might have looked: standing like a cow, and about the same size.
"Imagine a cow-sized, plant-eating reptile with a knobby skull and bony armor down its back," said co-author Linda Tsuji of the Royal Ontario Museum, who discovered the fossils in Niger along with Sidor and colleagues in 2003 and 2006.
Turner examined much of the skeleton of several individuals. In particular, four observations make the case, she said, that Bunostegos stood differently than all the rest, with the legs entirely beneath the body.
Starting at the shoulder joint, or the glenoid fossa, the orientation of it 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, too.
Meanwhile Bunostegos's 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 but then orient the forearm downward from the elbow.
But the humerus of Bunostegos has no twist suggesting that only if the elbow and shoulders were aligned under the body, could the foot actually reach the ground, 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 would not 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 does.
Finally, the ulna is longer than the humerus in Bunostegos, which is a common trait among non-sprawlers.
The study is published in the Journal of Vertebrate Paleontology.
