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New research has revealed how more upright limb postures helped ancient reptiles overcome the biomechanical constraints of body size, paving the way for the evolution of larger sizes in archosaurs鈥攁 group that includes crocodilians, dinosaurs and birds. The collaborative study involved the Royal Veterinary College (RVC), along with the Institute of Vertebrate Paleontology and Paleoanthropology (Beijing, China) and Clemson University (South Carolina, U.S.).

The paper, which has been in Science Advances, was conducted using computational modeling and simulation of hindlimb movements, muscle usage and thigh bone stresses across hindlimb postures and body sizes in juvenile to adult American alligators鈥攑art of an ancient reptile lineage that has its origin in the earliest Triassic period, around 250 million years ago.

Analyzing these simulation results, the research team, including Professor 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 alligators of different sizes generate the limb muscle forces and bone strength needed to support their weight.

The results show that as alligators become larger, their muscles and bones experience increasing mechanical stresses. However, when they adopt a more erect, upright posture with their legs positioned closer beneath the body, these stresses are reduced. It is this biomechanical relief that allowed some early reptiles to evolve to larger body sizes without overloading their limbs.

These findings were also tested using a scaled model of the extinct giant alligatoroid, Deinosuchus riograndensis, which lived alongside dinosaurs in the Late Cretaceous of Texas. This model allowed the research team to understand how Deinosuchus moved on land despite its enormous size鈥攅xceeding 3 metric tons, making it one of the largest crocodilians ever, and roughly three times heavier than even the largest saltwater crocodiles today.

By simulating limb biomechanics in both modern alligators and Deinosuchus, the study revealed that extremely large crocodilians may have lost the ability to fully lift themselves off the ground, relying instead on belly-dragging for land locomotion.

This analysis suggests that once an animal with a crocodilian-like body shape exceeds the size of adult American alligators (several hundred kilograms) and approaches the size of Deinosuchus, the mechanical constraints of muscle force and bone strength become too great for walking.

Professor John Hutchinson, professor of evolutionary biomechanics at the RVC, said, "Extinct animals often had forms and sizes unlike those of living animals.

"Science can indirectly test how those forms and sizes might have produced locomotion, and how that locomotion compares with that of living relatives. We thereby can learn more about the fundamental principles of locomotion than we can by purely studying living animals."

Dr. Masaya Iijima, associate researcher at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China and lead author of the study, said, "We have studied modern alligators for years, through both experiments and simulations to investigate the evolution of limb posture and its functional implications.

"To further explore the evolutionary link between limb posture and body size in four-limbed vertebrates, our next step is to analyze fossil evidence鈥攊ncluding skeletons and footprints鈥攖o reveal their evolutionary patterns."

Professor Richard Blob, alumni distinguished professor at Clemson University, South Carolina, U.S., said, "Giant body size puts extreme demands on organisms, but it has still evolved multiple times in Earth history.

"Understanding how organisms survive extremes is a powerful tool for understanding what factors place boundaries on biodiversity. Biomechanical analyses like ours can give insight into how organisms push those boundaries and help explain why we see the range of body designs that we do today."