Tetrapods were the earliest four-legged vertebrate animals who moved from water to land around 390 million years ago. This necessitated changes in their musculoskeletal systems to permit a terrestrial life. Forelimbs and hind limbs of the first tetrapods evolved to support more weight. Providing new insight into the evolution of how tetrapods took their first steps on land, a team of scientists from the National Institute for Mathematical and Biological Synthesis (NIMBioS) and Clemson University have evaluated the specific mechanisms that drove changes in bone function.
In order to understand the biology of fossilized animals, researchers often turn to living animals with similarities that help model how extinct animals moved.
‘The specific mechanisms that drove changes in bone function of tetrapods has been evaluated by researchers. They found that the forelimbs, compared to the hind limbs, had lower yield stresses, higher mechanical hardness, and a greater ability to withstand loads higher than normal.’
Tiger salamanders are particularly good organisms for studying how locomotion onto land evolved, as their anatomy and ecology is similar to the earliest tetrapods. Bones must regularly withstand a variety of different forces, or 'loads', from both the contraction of muscles and from interaction with the environment. Limb bones in particular must accommodate some of the highest forces.
Fossil records suggest that the forelimb and hind limb may have had different functions for walking on land, but the specific mechanisms that contributed to these differences are less known. The researchers wanted to test what factors could have driven diversity in skeletal design in the evolution of early tetrapods.
The mechanics of bone loading in the salamanders were tested in a variety of ways, including filming the salamanders as they walked across a custom-built platform that measured forces on the limb bones. A comparison of forelimbs and hind limbs and analysis of limb joints were conducted.
Mathematical models were used to evaluate how the limb bones were able to withstand the physical demands of walking on land. To assure a good test, salamanders that turned, stopped or fell on the platform or walked diagonally, for example, were excluded from the study. The study found that the forelimbs, compared to the hind limbs, had lower yield stresses, higher mechanical hardness, and a greater ability to withstand loads higher than normal.
Lead author Sandy Kawano, a postdoctoral fellow at NIMBioS, said, "These results offer new perspectives in modeling how tetrapods may have taken their first steps onto land, by considering the unique contributions of both the fore limbs and hind limbs."