A study that looked at the bones of a prereptile known as Bunostegos akokanensis, which lived about 260 million years ago, revealed that it may have stood upright on all fours. The findings suggest that the creature could be the earliest known animal to have done so.
The prereptile, which resembles a reptilian pit bull, belongs to the pareiasaur group, herbivores that lived during the Permian between 298 and 252 million years ago.
All known pareiasaurs that roamed the ancient supercontinent Pangea during this era were sprawlers with limbs that extended from the side of their body then continued slanting downward from the elbow, just like some of the lizards that exist today.
Researchers thought that the Bunostegos was also a sprawler, but the forelimbs of the animal suggest something else. The animal stood differently from its counterparts, having legs entirely beneath its body.
Study co-author Morgan Turner, whose research with colleagues was published in the Journal of Vertebrate Paleontology, expected that the Bunostegos would also be a sprawler but the creature's forelimbs tell something else. The features and elements of the forelimb bones of the Bunostegos do not allow a sprawling posture.
"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," said Turner, who was involved with the analysis of the creature's bones while studying at the University of Washington and is now a grad student at Brown University. At the time, many of the animals had upright or semi-upright hind limb posture.
The analysis allowed the researchers to determine what the animal might have looked like: it stood like a cow and was about the same size. The researchers also described the herbivorous reptile as having bony armor down its back and a knobby skull.
Evidence suggests that the creature faced long-term isolation and thrived in a harsh habitat with few water sources and plants. It was discovered by a team of paleontologists working in Niger in 2003 and 2006. Scientists suggest that the Bunostegos may have evolved its upright posture to accommodate its long journey, since an upright posture is known to be more energy-efficient compared with a sprawling posture.
Analysis also revealed that the Bunostegos retained some primitive characteristics that are present in older reptiles but not found in other pareisaurs. That lead researchers to conclude that the creature is a split-off from other creatures in its group and that it later independently evolved its distinct head.
