Pre-reptile likely first to stand upright on four legs

PROVIDENCE, R.I., Sept. 18 (UPI) — Researchers believe they’ve identified the first creature to ever walk upright on four legs.

First discovered in 2003, Bunostegos akokanensis is a pareiasaur parareptile, or pre-reptile, that looked like a cross between a cow and a rhino and wandered the supercontinent of Pangea some 260 million years ago. Scientists were only recently able to conduct an in-depth analysis of the species’ leg anatomy.

Their findings proved that — unlike the species’ relatives, whose legs bowed outward, sprawled liked a lizards — Bunostegos’ limbs stood rigid and erect.

“The elements and features within the forelimb bones won’t allow a sprawling posture,” researcher Morgan Turner said in a press release. “That is unique.”

Turner, who conducted the research while a student at the University of Washington but is now a grad student at Brown University, is the lead author of a new study on the species, published this week in the Journal of Vertebrate Paleontology.

Turner and her colleagues identified several design components that make Bunostegos’ forelimbs unique. To begin, its shoulder socked is angled straight down. Its humerus was unable to twist, a necessity for sprawlers. And its elbow joint works like a hinge, enabling a back and forth movement similar to the motion encouraged by the human knee.

Finally, its ulna is longer than its humerus — the case for most upright walkers.

“Many other sprawling four-legged animals have the reverse ratio,” Turner explained.

The discovery pushes back the arrival of upright four-legged posture among animals on the evolutionary timeline, but it doesn’t necessarily change the story of evolution. Non-sprawled walking posture evolved separately, several times over, among both reptiles and mammals.

Turner says she wouldn’t be surprised if scientists eventually uncover peers of Bunostegos with a similar stance and gait.

“Posture, from sprawling to upright, is not black or white, but instead is a gradient of forms,” Turner said. “There are many complexities about the evolution of posture and locomotion we are working to better understand every day. The anatomy of Bunostegos is unexpected, illuminating, and tells us we still have much to learn.”