For the first time, scientists have been able to reconstruct, the intricate three-dimensional structure of the backbone of early tetrapods, the earliest four-legged animals.
High-energy X-rays and a new data extraction protocol allowed the researchers to reconstruct the backbones of the 360 million year old fossils in exceptional detail and shed new light on how the first vertebrates moved from water onto land.
The international team of scientists was led by Stephanie E. Pierce from the Royal Veterinary College in London and Jennifer A. Clack from the University of Cambridge. It also comprised scientists from Uppsala University (Sweden) and the European Synchrotron Radiation Facility ESRF in Grenoble (France).
The tetrapods are four-limbed vertebrates, which are today represented by amphibians, reptiles, birds and mammals.
Around 400 million years ago, early tetrapods were the first vertebrates to make short excursions into shallower waters where they used their four limbs for moving around.
How this happened and how they then transferred to land is a subject of intense debate among palaeontologists and evolution biologists.
All tetrapods have a backbone, or vertebral column, which is a bony structure common to all other vertebrates including fish, from which tetrapods evolved. A backbone is formed from vertebrae connected in a row - from head to tail.
Unlike the backbone of living tetrapods (e.g. 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," lead author Stephanie Pierce said.
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.
The fossilised bones are embedded in rock so dense it absorbs most of the X-rays.
"Without the new method, it would not have been possible to reveal the elements of the spine in three dimensions with a resolution of 30 micrometres," co-author Sophie Sanchez from University of Uppsala and ESRF said.
In these high-resolution X-ray images, the 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.
"By understanding how each of the bones fit together we can begin to explore the mobility of the spine and test how it may have transferred forces between the limbs during the early stages of land movement,"
The study is published in the journal Nature.