A study on the shoulder and limb bones of the pre-reptile Bunostegos akokanensis has revealed that it may have been among the first animals to walk upright. Bunostegos belonged to the Pareiasaurs family, which were large, herbivorous reptiles. They roamed the Earth about 265 million years ago when the continents were combined into Pangaea.
The fossil itself isn’t new, it was discovered in a desert in Niger in 2006 but this is the first study on its limb morphology.
The team of biologists and paleontologists studied the orientation of several of bones in the limbs, shoulders and pelvis of the animal. The shoulder joint was faced downward which would mean the humorous (upper arm bone) could only have pointed down and been below the animal, similar to the orientation of a dog’s limb. The elbow joint also suggests an upright posture. It could only move forward and backwards like the movement of our knee.
The other known lizards of the time had very different limb morphology and mobility. The limbs extended out from their sides and they had much more mobility, which allowed they to walk in more of a sprawl like fashion. The image below is of another Pareiasaur present around the same time as Bunostegos. In this organism the legs point outward and then downward. It would also have had more mobility in its elbows and knees. This was a more typical orientation of reptile legs at the time.
So the question is; why did this limb orientation evolve in Bunostegos while the sprawling orientation was clearly so successful in all the other Pareiasaurs at the time? While we don’t know for sure, one hypothesis is because of the long trips it had to take. Bunostegos lived in a very dry and arid part of Niger and it had to walk long distances to obtain food and water. Its upright posture could have been more energy efficient in terms of walking than the sprawling.
There were probably some intermediates between sprawling posture and upright posture. The transition would not have occurred overnight and probably required several stages; the fossils of which have not been found.
“Posture, from sprawling to upright, is not black or white, but instead is a gradient of forms.” Said Morgan Turner, a graduate student at Brown University and head of the study.
Bunostegos is the oldest fossil ever found displaying this style of walking. This doesn’t necessarily mean it was the first though. There could have been others with legs like this at the time but fossils of these haven’t been found. The fossil record is very incomplete for a number of reasons.