How Changing Limb Posture Helped Ancient Reptiles Grow

New research has revealed how more upright limb postures helped ancient reptiles overcome biomechanical constraints on body size, paving the way for the evolution of larger sizes in archosaurs. Collaborative studies involved the Royal Veterinary College (RVC), as well as the Institute of Vertebrate Paleontology and Paleoanthropology (Beijing, China) and Clemson University (South Carolina, US).

The paper, published in Science advanceswas conducted using computational modeling and modeling of hindlimb movements, muscle use, and femoral bone tension in hindlimb postures and body size in juveniles for adult American fish.

Analyzing these simulation results, the research team, including John Hutchinson, Professor of Evolutionary Biomechanics at the RVC and Fellow of the Royal Society, explored how changes in hindlimb posture affect muscle forces and bone stresses, and how fish of different sizes develop limb muscle forces and bone strength to support their weight.

The results show that as fish get older, their muscles and bones experience increased mechanical stress. However, when they adopt a more upright, upright posture with the legs under the body, these stresses are reduced. It is this biomechanical relief that allowed some early reptiles to evolve to large body sizes without overburdening their limbs.

These findings were also tested using a miniature model of the extinct giant alligatorid, Deinosuchus riograndensis, which lived alongside dinosaurs in the Late Cretaceous of Texas. The model allowed the research team to understand how Dinosuchus moved on land despite its enormous size – at more than 3 metric tons, it is one of the largest crocodiles ever found, and nearly three times heavier than the largest saltwater crocodiles today.

By simulating the biomechanics of limbs in both modern alligators and Dinosuchus, the study revealed that the largest crocodilians had lost the ability to lift themselves off the ground entirely, relying on abdominal drag rather than landing.

This analysis suggests that once a crocodile-like body size exceeds the size of an adult American fish (several hundred kilograms) and approaches the size of Denosuchus, the mechanical constraints of muscle strength and bone strength become too great to walk.

Professor John Hutchinson, Professor of Evolutionary Biomechanics at RVC, said: “Extinct animals often have shapes and sizes unlike living animals.

“Science can indirectly test how these shapes and sizes might have developed locomotion, and how locomotion compares with living relatives. We can learn more about the fundamentals of locomotion than we can by studying living animals this way.”

“We have studied modern fish for years, both experimentally and through simulations, to investigate limb posture and its functional implications,” said Dr Masiya Iijima, an associate researcher at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China and the study’s lead author.

“To further explore the evolutionary links between limb posture and body size in four-limbed vertebrates, our next step is to analyze the fossil evidence—including skeletons and footprints—to reveal their evolutionary patterns.”

“Large body size places extreme demands on organisms, but it has still evolved many times in Earth’s history,” said Professor Richard Blub, Alumni, South Carolina, Distinguished Professor at Clemson University in the US.

“Understanding how organisms survive extremes is a powerful tool for understanding what factors impose limits on biodiversity. Biomechanical analyzes like ours can provide insight into how organisms push these limits and help explain why we see the limits of body design that we do today.”