Bunostegos akokanensis – a Permian cow-sized, herbivorous reptile with a knobby skull and bony armor down its back – is the oldest known creature to have walked upright on all fours, according to a team of paleontologists led by Prof. Christian Sidor of the University of Washington, Seattle.
Artist’s rendering of Bunostegos. Image credit: Marc Boulay.
Bunostegos akokanensis belongs to Pareiasauria (pareiasaurs), a group of plant-eating reptiles that lived in the arid regions of the supercontinent Pangaea (modern-day Europe, Asia, South America, and Africa) during the middle to late Permian, about 265 – 252 million years ago.
Most pareiasaurs had bony knobs on their skulls, but Bunostegos akokanensis sported the largest, most bulbous ones ever discovered. In life, these were probably skin-covered horns like those on the heads of modern giraffes.
To date all of the known pareiasaurs were sprawlers whose limbs would jut out from the side of the body and then continue out or slant down from the elbow.
Prof. Sidor and co-authors expected Bunostegos akokanensis 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, but what’s interesting and special about Bunostegos akokanensis is the forelimb, in that its anatomy is sprawling-precluding and seemingly directed underneath its body – unlike anything else at the time,” said Morgan Turner of Brown University, team member and lead author on the study published in the Journal of Vertebrate Paleontology.
“The elements and features within the forelimb bones won’t allow a sprawling posture. That is unique.”
The findings allowed the team to characterize how Bunostegos akokanensis might have looked: standing like a cow, and about the same size.
The paleontologists examined much of the skeleton of several individuals.
“The findings that matter most, however, are all in the forelimbs,” Turner explained.
“In particular, four observations make the case that Bunostegos stood differently than all the rest, with the legs entirely beneath the body.”
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Morgan L. Turner et al. The vertebrate fauna of the upper Permian of Niger—IX. The appendicular skeleton of Bunostegos akokanensis (Parareptilia: Pareiasauria). Journal of Vertebrate Paleontology, published online September 11, 2015; doi: 10.1080/02724634.2014.994746
