One of the studied animals was a fierce-looking, toothy beast known as Ichthyostega, which lived 374 - 359 million years ago and was a transitional species between fish and terrestrial animals.
Ichthyostega is thought to have navigated through shallow water in swamps, probably lured by food.
High-energy X-rays and a new data extraction protocol allowed the researchers to reconstruct the backbones of the fossils in exceptional detail and shed new light on how the first vertebrates moved from water onto land.
The study published in the journal Nature was led by Stephanie E Pierce from The Royal Veterinary College in London and Jennifer A Clack from the University of Cambridge.
Researchers found that around 400 million years ago, early tetrapods were the first vertebrates to make short excursions into shallower waters where they used their four limbs to move around.
Unlike the backbone of living tetrapods like humans, in which each vertebra is composed of only one bone, early tetrapods had vertebrae made up of multiple parts.
"For more than 100 years, early tetrapods were thought to have vertebrae composed of three sets of bones - one bone in front, one on top, and a pair behind. But, by peering inside the fossils using synchrotron X-rays we have discovered that this traditional view literally got it back-to-front," said Pierce in a statement.
For the analysis, the European Synchrotron Radiation Facility (ESRF) in France, where the three fossil fragments were scanned with X-rays, applied a data extraction method to reveal tiny details of fossil bones buried deep inside the rock matrix.
Scientists discovered that what was thought to be the first bone - known as the intercentrum - is actually the last in the series.
And, although this might seem like a trivial oversight, this re-arrangement in vertebral structure has over-arching ramifications for the functional evolution of the tetrapod backbone.
