Scientists believe that a pre-reptile, which lived 260 million years ago, may have been the first animal to walk on four legs.
Analysis of fossils from the Bunostegos akokanensis, which were discovered in Niger between 2003 and 2006, show that its forelimbs were uniquely designed to allow the animal to stand upright in the manner of a cow.
The fossils belong to a group of pre-reptiles known as pareiasaurs—the first group of large land vertebrates which flourished during the Permian period, a geological period which lasted from 299 million to 251 million years ago. All of the known pareiasaurs were sprawlers, with limbs similar to modern lizards that would jut out from the sides of the body.
However, B. akokanensis had a number of distinct features which means it would have resembled "a cow-sized, plant-eating reptile with a knobby skull and bony armor down its back," according to Linda Tsuji from the Royal Ontario Museum, a co-author of the study published in the Journal of Vertebrate Paelontology.
The fossils, show that the humerus of B. akokanensis was straight, in contrast to the twisted humerus possessed by sprawlers. Its shoulder joint points directly downwards, suggesting a straight alignment of bones underneath.
The creature's elbow joint is a hinge joint—similar to the human knee, meaning it can only swing back and forth and precluding the sideways movement characteristic of spralwers. Finally, the fossil's ulna—a bone in its forearm—is longer than the humerus, which the researchers said is a common-trait in non-sprawling animals.
Researchers believe that the pre-reptile's upright posture could have helped it in finding food in its arid environs. Compared to sprawling, walking upright on four legs is associated with a more energy efficient posture and may have helped B. akokanensis conserve energy on long journeys between meals.