Reptile Relative Was Among the First Animals to Walk On Upright Legs

In the ongoing history of evolutionary change, there are few transformations as celebrated as the time when fossil humans gave up a life in the trees for one walking upright on the ground. It’s so special to us that, rightly or wrongly, the ability to strut about on two pillar-like legs has often been taken as the de facto evidence for membership in the hominin family. We often forget that other creatures pulled the same feat hundreds of millions of years before us.

Way back in Triassic time, when our mammalian ancestors were scuttling around on bowed limbs, both dinosaurs and extinct crocodile cousins called poposaurs chased around the forests and floodplains on two legs held directly beneath their hips. They beat us by over 230 million years. And if we’re talking about creatures held up on column-like legs irrespective of bipedal posture, a strange relative of early relatives has pushed back the origin of upright walking much further back into prehistory.

Given the name Bunostegos akokanensis, the odd critter was initially described by paleontologist Christian Sidor and colleagues in 2003 on the basis of skull pieces found in the 265-to-252-million-year-old rock of Niger. But the same site held even more of the animal’s skeleton: elements of the limbs, shoulders, hips, and spine of at least nine individual Bunostegos, the largest of which indicate this species could reach about eight feet in length. Together, these pieces reveal that Bunostegos was one of the pareiasaurs, or a lineage of archaic reptile relatives that fell outside the brackets which encompassed the ancestors of lizards, turtles, dinosaurs, and their kin.

Up until now, pareiasaurs seemed to have shared the same bow-legged posture. While their hind limbs were a little closer to being straight up-and-down, the parareptiles’ forelimbs splayed out to the side in a sprawling posture that makes their reconstructed skeletons look as if they’re always in the middle of a push-up. But Bunostegos was different. In a description of the animals postcrania, Morgan Turner, Sidor, and coauthors propose that this pareiasaur held itself off the ground on nearly upright arms and legs.

The evidence that Bunostegos was unusual compared to its relatives comes down to three points of evidence. In Bunostegos, Turner and colleagues write, the part of the shoulder where the upper arm bone articulated is oriented more downward and to the back than the to-the-side position in other pareiasaurs, hinting at a more vertical articulation. That upper arm bone, the humerus, doesn’t fit into the traditional sprawling posture, and the elbow joint indicates movement in a front-to-back orientation expected of an animal holding itself on columnar limbs. So while almost-upright hindlimbs were typical for pareiasaurs, Bunostegos was one step beyond in having all four limbs set up straight.

Similar cases might turn up as paleontologists sift through the Permian fossil record, but, as it currently stands, Bunostegos was unique among the creatures of its time. The question is why. While that puzzle is difficult to solve without a richer understanding of the parareptile’s relatives and the animal’s natural history, Turner and coauthors speculate that the search for scarce food may have had something to do with the transformation.

During the time of Bunostegos, Niger was a desert where there may have been large swaths of relatively barren ground between vital patches of sustaining plants. Standing tall on upright legs is a much more efficient way to move at speed, and so, over generations, pareiasaurs with increasingly-upright arms may have been able to trot from one cluster of greens to another more efficiently and, being in better condition, leave behind more offspring than the sprawlers. Whether or not this was actually the case will depend on future research, but, for now, one thing is clear—Bunostegos was one of the first creatures to stand tall.

Reference:

Turner, M., Tsuji, L., Ide, O., Sidor, C. 2015. The vertebrate fauna of the upper Permian of Niger – IX. – The appendicular skeleton of Bunostegos akokanensis (Parareptilia: Pareiasauria)The vertebrate fauna of the upper Permian of Niger – IX. – The appendicular skeleton of Bunostegos akokanensis (Parareptilia: Pareiasauria)The vertebrate fauna of the upper Permian of Niger – IX. – The appendicular skeleton of Bunostegos akokanensis (Parareptilia: Pareiasauria). Journal of Vertebrate Paleontology. doi: 10.1080/02724634.2014.994746