The first four-legged animals dragged themselves across the ground like seals, a reconstruction of their backbones has shown.
An examination at the European Synchrotron Radiation Facility (ESRF) in Grenoble using high-energy X-rays has enabled a detailed reconstruction of the backbones of 360-million-year-old fossils and shed new light on how the first vertebrates moved from water onto land.
Around 400 million years ago, early tetrapods started visiting shallower waters, where they used their four limbs for moving around. But how this happened. and how they then shifted to the land, has been subject to debate.
All tetrapods have a backbone, or vertebral column; but, unlike the backbone of living tetrapods, 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," says Stephanie Pierce, lead author of the report.
"But, by peering inside the fossils using synchrotron X-rays we have discovered that this traditional view literally got it back-to-front."
Scanning at the European Synchrotron Radiation Facility (ESRF) revealed tiny details of fossil bones buried deep inside the rock. The high-resolution X-ray images show that what was thought to be the first bone - known as the intercentrum - is actually the last. This has important ramifications for the functional evolution of the tetrapod backbone, says Pierce.
"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," she says.
The team also discovered that one of the animals - known as Ichthyostega - was also found to have a collection of hitherto unknown skeletal features, including a string of bones extending down the middle of its chest.
These appear to be an early version of a bony sternum, strengthening the ribcage and allowing Ichthyostega to support its body weight on its chest. It probably moved by dragging itself across flat ground using 'crutching' motions of its front legs - much like a mudskipper or seal.
"The results of this study force us to re-write the textbook on backbone evolution in the earliest limbed animals," says Pierce.