How Salamander might be a missing link to our legs

The tiger salamander might provide some clues. A new study from a team of scientists from the National Institute for Mathematical and Biological Synthesis (NIMBioS) and Clemson University evaluates what mechanisms drive diversity in bone function, providing new insight into the evolution of how tetrapods--the earliest four-legged vertebrate animals--took their first steps on land.

In order to understand the biology of fossilized animals, researchers often turn to living animals with similarities that help model how extinct animals moved. 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 an 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.

"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 forelimbs and hind limbs, " said lead author Sandy Kawano, a postdoctoral fellow at NIMBioS.

S. M. Kawano, D. R. Economy, M. S. Kennedy, D. Dean, R. W. Blob. Comparative limb bone loading in the humerus and femur of the tiger salamander Ambystoma tigrinum: testing the 'mixed-chain' hypothesis for skeletal safety factors. Journal of Experimental Biology, 2015; DOI: 10.1242/jeb.125799