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September 17, 2015

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Cow-sized beast may be earliest known to walk upright on all fours

Sept. 17, 2015
Courtesy of Brown University
and World Science staff

A cow-sized an­i­mal de­scribed as a pre-rep­tile may have been the ear­li­est known crea­ture to walk on all fours, ac­cord­ing to sci­en­tists.

A newly pub­lished anal­y­sis dates the bones of the an­i­mal, Bunos­te­gos akoka­nen­sis, to 260 mil­lion years ago—around 10 mil­lion years be­fore the age of the di­no­saurs.

Artist's reconstruction of Bunos­te­gos akoka­nen­sis (Cre­dit: Mor­gan Turn­er)


“Imag­ine a cow-sized, plant-eating rep­tile with a knob­by skull and bony ar­mor down its back,” said Lin­da Tsuji of the Roy­al On­tar­i­o Mu­se­um, who co-discovered the fos­sils in Ni­ger in 2003 and 2006.

A re­port on the find­ings by Tsuji and oth­ers de­scribes Bu­no­s­te­gos as a pareiasaur, one of a group of an­i­mals that roved Pangea—a land con­sist­ing of al­most all the land mass­es of Earth crammed in­to one “su­per­con­ti­nent.”

The known pa­rei­a­saurs, how­ev­er, were “sprawlers” whose limbs would jut out from the side of the body and then con­tin­ue out or slant down from the el­bow, like some mod­ern lizards, ac­cord­ing to the re­search­ers.

Mor­gan Turn­er, lead au­thor of the re­port and cur­rently a grad­u­ate stu­dent at Brown Un­ivers­ity in Rho­de Is­land, said she ex­pected the same of Bu­no­s­te­gos, but the bones tell a dif­fer­ent sto­ry.

“A lot of the an­i­mals that lived around the time had a si­m­i­lar up­right or semi-up­right hind limb pos­ture,” she ex­plained. But the fore­limb of Bu­no­s­te­gos is “seem­ingly di­rect­ed un­derneath its bod­y… The el­e­ments and fea­tures with­in the fore­limb bones won’t al­low a sprawl­ing pos­ture.”

Artist's reconstruc­tion of Brady­saur­us see­le­yi, a more pri­mi­tive pa­rei­a­saur thought to have a more sprawl­ing pos­ture. (Cre­dit: No­bu Ta­mu­ra)


Among oth­er things, the shoul­der joint faces down; the hu­mer­us, the bone run­ning from shoul­der to el­bow, is­n’t twisted as it is in “sprawl­ers”; the el­bow has lim­it­ed range, block­ing the fore­arm from swing­ing side­ways; and the fore­arm bones are long­er than the hu­mer­us, some­thing com­mon among non-sprawl­ers.

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

Bu­no­s­te­gos lived in a tough hab­i­tat some­what apart from oth­er pa­rei­a­saurs, she added, and the up­right pos­ture may have had some­thing to do with this. Ni­ger was a dry place then, with plants and wa­ter sources few and far be­tween. Sci­en­tists have as­so­ci­at­ed walk­ing up­right on all fours with more en­er­gy ef­fi­cien­cy than sprawl­ing. For the long jour­neys be­tween meals, Turn­er said, the up­right pos­ture might have been cru­cial for sur­viv­al.

Bu­no­s­te­gos seems to push back the clock on when the up­right pos­ture shows up in ev­o­lu­tion, Turn­er said. But she added that she would­n’t be sur­prised if oth­er an­i­mals of the time turn out to have si­m­i­lar­i­ties to this pos­ture, which has evolved in­de­pend­ently in rep­tiles and mam­mals sev­er­al times.

“Pos­ture, from sprawl­ing to up­right, is not black or white,” but comes in end­less shades, Turn­er said. “There are many com­plex­i­ties about the ev­o­lu­tion of pos­ture and lo­co­mo­tion we are work­ing to bet­ter un­der­stand eve­ry day.”


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A cow-sized animal described as a pre-reptile may have been the earliest known creature to walk on all fours, according to scientists. A newly published analysis dates the bones of the animal, Bunostegos akokanensis, to 260 million years ago—around 10 million years before the age of the dinosaurs dawned. “Imagine a cow-sized, plant-eating reptile with a knobby skull and bony armor down its back,” said Linda Tsuji of the Royal Ontario Museum, who co-discovered the fossils in Niger in 2003 and 2006. A report on the findings by Tsuji and others describes Bunostegos as a pareiasaur, one of a group of animals that roved Pangea—a land consisting of almost all the land masses of Earth crammed into one “supercontinent.” The known pareiasaurs, however, were “sprawlers” whose limbs would jut out from the side of the body and then continue out or slant down from the elbow, like some modern lizards, according to the researchers. Morgan Turner, lead author of the report and currently a graduate student at Brown University in Rhode Island, said she expected the same of Bunostegos, but the bones tell a different story. “A lot of the animals that lived around the time had a similar upright or semi-upright hind limb posture,” she explained. But the forelimb of Bunostegos is “seemingly directed underneath its body… The elements and features within the forelimb bones won’t allow a sprawling posture.” Among other things, the shoulder joint faces down; the humerus, the bone running from shoulder to elbow, isn’t twisted as it is in “sprawlers”; the elbow has limited range, blocking the forearm from swinging sideways; and the forearm bones are longer than the shoulder-to-elbow bone, the humerus. The latter is a common trait among non-sprawlers, Turner said. “Many other sprawling 4-legged animals have the reverse ratio,” she explained. Bunostegos also lived in a tough habitat somewhat apart from other pareiasaurs, she added, and the upright posture may have had something to do with this. Niger was a dry place then, with plants and water sources few and far between. Scientists have associated walking upright on all fours with more energy efficiency than sprawling. For the long journeys between meals, Turner said, the upright posture might have been crucial for survival. Bunostegos seems to push back the clock on when the upright posture shows up in evolution, Turner said. But she added that she wouldn’t be surprised if other animals of the time turn out to have similarities to this posture, which has evolved independently in reptiles and mammals several times. “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.”