By altering their posture, ancient reptiles grew larger.

Some reptiles started experimenting with a novel mode of locomotion some 250 million years ago as life was struggling to recover from the largest mass extinction on Earth.

They began to stand straighter, pulling their legs under their body, rather than sprawled like lizards with their limbs spread wide. The evolutionary path would be altered by this small shift in posture, allowing reptiles to develop into enormous creatures like crocodiles, dinosaurs, and birds.

Everything changed when I stood tall.

According to a recent study, ancient reptiles were able to overcome the mechanical constraints of body size by adopting more upright limb positions.

Animals' muscles and bones must endure more strain as they get bigger. For early reptiles, this meant that their limbs could only sustain their weight up to a certain size. They lessened those strains and opened the door to much larger forms by putting their legs closer beneath their body.

Reptiles' altered stance

Researchers simulated how a reptile's position influences the stresses in its hind legs using sophisticated computer models.

The findings demonstrated that alligators' bones and muscles undergo reduced mechanical loads when they adopt a more erect stance, which facilitates the carrying of bigger bodies.

Some reptilian lineages may have grown to huge sizes while others remained tiny and sprawled due to this biomechanical adjustment, which may have been the evolutionary key.

Takeaways from actual alligators

To investigate this historic transition, the research team looked to American alligators. These reptiles provide a living window into the anatomy of early archosaurs since they are members of an evolutionary line that dates back to the Triassic.

Scientists studied thigh bone stresses, muscle activity, and hindlimb movement in people of all body sizes, from children to adults, using computer simulations.

The data showed a distinct pattern: mechanical forces on the alligators' limbs dramatically increased as they matured. However, those strains were reduced by standing more erect.

The fundamentals of movement

The success of the archosaurs, the group that eventually gave rise to crocodiles, dinosaurs, and modern birds, may have been influenced by this little evolutionary modification. John Hutchinson, a professor of evolutionary biomechanics at the Royal Veterinary College, is a co-author of the study.

Reconstructing a prehistoric giant The team constructed a biomechanical model of Deinosuchus riograndensis, a massive alligatoroid that roamed what is now Texas during the Late Cretaceous. Weighing more than three metric tonnes, Deinosuchus dwarfed modern crocodiles, reaching roughly three times the mass of the largest living saltwater crocodiles. According to Professor Hutchinson, "Science can indirectly test how those forms and sizes might have produced locomotion, and how that locomotion compares with that of living relatives."

Such enormous animals could have found it difficult to raise their bellies entirely off the ground, according to the simulations. Rather, they probably moved on land using a belly-dragging stride, saving their power for the water, where buoyancy made their weight easier to bear.

This discovery shows that reptile biomechanics can only go so far. Animals that weigh more than several hundred kilograms—the size of current alligators—experience stresses on their limbs that are too large to support full walking.

Effects of reptiles' altered posture

The primary author of the study is Dr. Masaya Iijima, an associate researcher at the Institute of Vertebrate Palaeontology and Paleoanthropology in Beijing, China.

"We have spent years researching the evolution of limb posture and its functional implications in modern alligators using both simulations and experiments," Dr. Iijima stated.

"Our next step is to analyse fossil evidence, such as skeletons and footprints, to reveal their evolutionary patterns in order to further explore the evolutionary link between limb posture and body size in four-limbed vertebrates." Fossil data may reveal the frequency and timing of reptiles' transition to a more erect posture.

The towering sauropods, armoured ankylosaurs, and quick-moving theropods that ruled later environments may have been made possible by this shift in posture, which may have been a pivotal moment in the evolution of vertebrates.

The legacy of upright posture

Every larger species eventually reaches a point where biology and physics meet. Bones can withstand a certain amount of stress before failing, and muscles can only produce a certain amount of force.

Scientists can learn more about why certain species grow into giants while others remain small by examining these boundaries. Richard Blob, a professor at Clemson University in South Carolina, is a co-author of the study.

According to Professor Blob, "giant body size puts extreme demands on organisms, but it has still evolved multiple times in Earth history."

"A powerful tool for understanding what factors place boundaries on biodiversity is understanding how organisms survive extremes."

"Biomechanical analyses like ours can help explain why we see the range of body designs that we do today and provide insight into how organisms push those boundaries."