The first vertebrates made the monumental transition from water to land roughly 390 million years ago, but it required some hefty evolutionary changes. For tiger salamanders, say researchers, that meant losing their sea appendages and developing forelimbs and hind limbs that force them to walk with a kind of strut that, upon closer examination, is providing some clues as to how tetrapods – the first four-legged vertebrate animals – evolved.

For their study, researchers researchers from the National Institute for Mathematical and Biological Synthesis (NIMBioS) and Clemson University discovered were interested in what mechanisms drive diversity in bone function. They turned to salamanders because their anatomy and ecology resembles that of the earliest tetrapods, whose musculoskeletal systems would have had to evolved in order to support their body mass when walking on land. (Scroll to read more...)

To test the mechanics of bone loading in salamanders, researchers filmed them as they walked across a custom-built platform that measured the gravitational forces their hind and forelimbs endure. With the additon of a limb joint analysis and comparison, researchers were able to estimate how limb bones are able to withstand the added pressure of walking on land. Terrestrial species are regularly subjected to a variety of different forces, or loads, when interacting with their environment. They must be equipped with specialized limb bones that can accommodate large loads and facilitate muscle contraction.

Overall, the researchers evaluation revealed that salamander forelimbs can withstand heavier loads than hind limbs. Additionally, their forelimbs have a higher mechanical hardness, which simply means they can better resist deformation from greater weight.  

"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," Sandy Kawano, lead author and a postdoctoral fellow at NIMBioS, explained in a news release.

Their study was recently published in the Journal of Experimental Biology. 

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