Posture Is Everything

An erect mode of locomotion is far more efficient than sprawling or semi erect modes

Forms of locomotion are incredibly diverse. Centipedes are propelled by waves of scuttling legs while snakes use muscles along the length of their bodies and friction to slither, sometimes at great pace.

Birds take to the wing, while fish use fin flicks to dart and shoal around reefs and in the deep ocean.

Earthbound creatures with four limbs by contrast seem rather mundane. Yet even here there is a surprising amount of diversity due to the different ways the limbs are connected to the body.

Some creatures, such as monitor lizards, have sprawling limbs which splay out on either side of the body, producing a distinctive gait so the body waggles from side to side. Others have legs which sit directly beneath the body. The first tetrapods, over tens of millions of years possessed sprawling limbs. This made movement slow as the torsion placed on the rib cage meant that they could only take breaths in between steps, requiring three feet on the ground at all times to support weight, moving one foot at a time.

The development of an erect posture with legs positioned below the body was revolutionary; allowing the dinosaurs to become fleet-footed, dominant predators. A new fossil discovery, however, shows that an erect placement of the limbs evolved far earlier in time than previously thought. The study focused on Bunostegos akokanensis.

Part of a group of Permian reptiles known as the pareiasaurs, members of this group described previously possessed sprawling limbs, but a re-examination of the skeletons of several individuals showed that Bunostegos had erect limbs positioned beneath the body.

The relationships between the bones of the limbs of Bunostegos akokanensis

'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 its anatomy is sprawling, and seemingly directed underneath its body, unlike anything else at the time,' said Morgan Turner from Brown University. 'That is unique.'

The shoulder joint was downward-facing. As a result the humerus would have been vertically oriented underneath and restricted from sprawling sideways. Additionally, in sprawlers the twist is what allows the humerus to jut out to the side at the shoulder and orient the forearm downwards from the elbow. Yet 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.

An artist's impression of Bunostegos akokanensis

The elbow joint is also telling. Unlike in sprawling pareiasaurs, which had considerable mobility at the elbow, the movement of the elbow in Bunostegos is more limited. The way the radius and ulna 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 backwards and forwards in a similar fashion to the human knee. The ulna itself is also longer than the humerus, whilst in sprawling pareiasaurs it is the other way round. Exactly why Bunostegos walked with an erect posture is another matter.

Bunostegos came from what would become modern day Niger. Its homeland would have been equally arid. An erect posture would have given Bunostegos greater mobility and efficiency in a land where water holes and plants would have been few and far between.

'Posture, from sprawling to upright, is not black or white, but instead is a gradient of forms,' concluded Turner. 'There are many complexities about the evolution of posture and locomotion we are working on to better understand every day. The anatomy of Bunostegos is unexpected, illuminating, and tells us we still have much to learn.'