The first known animal to have walked on all fours was a hideous cow-like creature that lived 260 million years ago.
About the size of a domestic cow, the plant-eater had bulbous tumour-like growths sprouting from its head and bony armour down its back.
The first known animal to have walked on all fours was a hideous cow-like creature that lived 260 million years ago. Dubbed Bunostegos akokanensis, the knobby-faced 'pre-reptile' was unlike any other animal that roamed the supercontinent Pangea. All other creatures found in the area from this time were known to be 'sprawlers'
Its fossils were first discovered in the arid landscape of the Sahara Desert of northern Niger in 2013.
Now, a new study of its skeletal structure suggest it raised its body off the ground to walk similar to the way many creatures do today.
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'.
This means their limbs would jut out from the side of the body and then continue out or slant down from the elbow, similar to some modern lizards.
A cow-like reptile that may have been one of nature's ugliest beasts roamed an isolated desert before the age of the dinosaurs. The creature's genus name - bunostegos - means 'knobby roof' and gives a clue to its appearance. The plant-eater had bulbous tumour-like growths sprouting from its head
Morgan Turner, lead author of the study in the Journal of Vertebrate Paleontology, expected Bunostegos would be a sprawler, too - but the bones of the animal's forelimbs tell a different story.
'A lot of the animals that lived around the time had a similar upright or semi-upright hind limb posture,' said Turner, who conducted the study with Christian Sidor at Washington University.
BUNOSTEGOS AKOKANESIS
A cow-like reptile that may have been one of nature's ugliest beasts roamed an isolated desert before the age of the dinosaurs.
The creature's genus name - bunostegos - means 'knobby roof' and gives a clue to its appearance.
About the size of a domestic cow, the plant-eater had bulbous tumour-like growths sprouting from its head and bony armour down its back.
Fossils from bunostegos dating back around 260 million years to the Permian era were found in what is now northern Niger in Africa
Back then the Earth was dominated by a single continent called Pangea. Bunostegos lived in an isolated desert in the middle of Pangea with unique fauna.
Cousins of the creature have previously been unearthed that also had bony knobs on their skulls, but those of the bunostegos are the largest and most bulbous ever discovered.
The latest study found the creature walked on all fours. The long bone in all of its limbs, named the humerus, did not have a structure called a twist that would allow the animal to move flat on the ground
The animal’s shoulder was also not able to move in a way that allowed the creature to sprawl on its stomach. And the elbow of all four limbs would not allow the animal to sprawl on the ground.
'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'
'The elements and features within the forelimb bones won't allow a sprawling posture. That is unique.'
Instead, the researchers believe the hideous creature stood like a cow, and was about the same size.
'Imagine a cow-sized, plant-eating reptile with a knobby skull and bony armour down its back,' said co-author Linda Tsuji of the Royal Ontario Museum, who discovered the fossils in Niger in 2003 and 2006.
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 the bone running from shoulder to elbow, known as the humerus, would be vertical underneath.
This 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 movement at the elbow, the movement of Bunostegos's elbow is more limited.
The way the 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 does.
Finally, the forearm is longer than the humerus in Bunostegos, which is a common trait among non-sprawlers, Turner said.
Pictured are the bones that revealed the creature couldn't sprawl. 1) the shoulder 2) the humerus 3) the elbow joint 4) the radius and ulna. The animal’s shoulder was also not able to move in a way that allowed the creature to sprawl on its stomach. And the elbow of all four limbs would not allow the animal to sprawl on the ground
'Many other sprawling four-legged animals have the reverse ratio,' she said.
The idea that Bunostegos would be an outlier in terms of its posture matches well with the idea that it was of an outlier in its choice of habitat.
'Bunostegos was an isolated pareiasaur,' explained Turner.
Scientists have associated walking upright on all fours with a more energy efficient posture than sprawling.
For the long journeys between meals, Turner said, the upright posture might have been necessary for survival.
The importance of such an early example of the upright posture is that Bunostegos dates very far back on the evolutionary tree, pushing back the clock on when this posture shows up in evolution.
But Turner said she wouldn't be surprised if other animals of the time are eventually also found to have similarities to this posture, which evolved independently in reptiles and mammals several times over the eras.
'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.'
The creature's fossils were first discovered in the arid landscape of the Sahara Desert of northern Niger in 2013. Now, a new study suggests it raised its body off the ground
Mariana Rano
Jacob Dunn
