An article published in the Journal of Vertebrate Paleontology recently states that paleontologists have found the oldest animal to stand upright on four legs. The animal is named Bunostegos akokanensis and lived 260 million years ago.
Bunostegos was an herbivore and about the size of a cow. It is the first known Pareiasaur to stand upright- all other known Pareiasaurs sprawled out, much like a lizard. The main differences observed in Bunostegos are in the animal's forelimbs. The shoulder joint was orientated vertically under its body, thus enabling it to stand upright.
Bunostegos was discovered in Niger back in 2003. 260 million years ago, Niger was very arid. Paleontologists think that Bunostegos had upright legs in order to walk more efficiently while searching for food in the harsh environment.