Researchers have found out a cow-sized reptile that lived 260 million years ago and likely stood upright on all four legs, 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 it's anatomy is sprawling-precluding and seemingly directed underneath its body - unlike anything else at the time," Morgan Turner, lead author of the new study, said.
Turner analysed the remains of the creature while working at the University of Washington's Burke Museum of Natural History and Culture.
To date, all known pareiasaurs that roved the super-continent of Pangea in the Permian era a quarter billion years ago, were "sprawlers" whose limbs would jut out from the side of the body and then continue or slant down from the elbow, like in some modern lizards.
"The elements and features within the forelimb bones won't allow a sprawling posture. That is unique," said Turner, who did the analysis under supervision of Professor Christian Sidor while he was a student at the University of Washington.
Now a graduate student at Brown University, Turner, along with Sidor and co-authors, characterised how Bunostegos might have looked: standing like a cow, and about the same size.
Linda Tsuji of the Royal Ontario Museum, who discovered the fossils along with Sidor and colleagues in 2003 and 2006, said the creature was like a "cow-sized, plant-eating reptile with a knobby skull and bony armour down its back."
After examining the skeletons of several creatures, Turner found out that the Bunostegos stood differently from the rest as it had legs entirely beneath the body.
Starting at the shoulder joint, or the glenoid fossa, the structure of the creature 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's humerus is not twisted like those of sprawlers. In a sprawler, the twist is what allowed the humerus to jut out to the side at the shoulder but 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.
In the final study, Turner found that the ulna is longer than the humerus in Bunostegos, a common trait among non-sprawlers.
Turner's study on Bunostegos has been published in the Journal of Vertebrate Paleontology.