Cow-Size Pre-Reptile May Have Been First Animal to Walk on Four Legs

Bunostegos akokaensis
An artist's rendering of Bunostegos akokaensis. This cow-like creature dates back 260 million years, and scientists believe it may have been the first animal to walk upright on all fours. Morgan Turner

Scientists believe that a pre-reptile that lived an estimated 260 million years ago may have been the first animal to walk on four legs.

Analysis of fossils from the Bunostegos akokanensis show that its forelimbs were uniquely designed to allow the animal to stand upright on all fours. Its body shape was likely cow-like, according to a new paper recently published in the Journal of Vertebrate Paelontology.

The fossils, which were discovered in Niger between 2003 and 2006, belong to a group of pre-reptiles known as pareiasaurs. These are the first group of large land vertebrates, and flourished during the Permian period, a geological period that 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," said Linda Tsuji from the Royal Ontario Museum, a co-author of the recent study, in a press release.

For example, the fossil study shows that the humerus (the long bone in the forelimb that runs from the shoulder to the elbow) 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. In addition, the creature's elbow joint is a hinge joint, similar to the human knee, meaning it can only swing back and forth and precludes the sideways movement characteristic of spralwers. Finally, the fossil's ulna bone (also in the forelimb, running from elbow to wrist) 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 might have been an evolutionary in advantage in what been very arid environs at the time. 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 rare meals.