Research published today in the journal Nature documents, for the first time, the intricate three-dimensional structure of the backbone in the earliest four-legged animals (tetrapods). The international team of scientists, led by Dr Stephanie Pierce from the University’s Zoology Department and the Royal Veterinary College and her Cambridge colleague Professor Jennifer Clack, bombarded 360 million year old early tetrapod fossils with high energy synchrotron radiation. The resulting high resolution X-ray images allowed the researchers to reconstruct the backbones of the extinct animals in exceptional detail.
Research has shown that the very earliest tetrapods were wholly aquatic and quite unsuited to life on land. This is in contrast to the earlier view that fish had first invaded the land — either in search of prey (like modern mudskippers) or to find water when the pond they lived in dried out — and later evolved legs, lungs, etc.
By the late Devonian, land plants had stabilized freshwater habitats, allowing the first wetland ecosystems to develop, with increasingly complex food webs that afforded new opportunities. Freshwater habitats were not the only places to find water filled with organic matter and choked with plants with dense vegetation near the water's edge. Swampy habitats like shallow wetlands, coastal lagoons and large brackish river deltas also existed at this time, and there is much to suggest that this is the kind of environment in which the tetrapods evolved. Early fossil tetrapods have been found in marine sediments, and because fossils of primitive tetrapods in general are found scattered all around the world, they must have spread by following the coastal lines.
The backbone, also known as the spine or vertebral column, is a bony structure found in all tetrapods, along with other vertebrates such as fish. It is formed from many elements or vertebrae all 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," said Pierce.
For the analysis, the European Synchrotron Radiation Facility (ESRF) in France, where the three fossil fragments were scanned with X-rays, used a new protocol to reveal tiny details of the fossil bones buried deep inside the rock matrix.
Using this new technology, the team of 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," explains Pierce.
But, the findings don’t end there. One of the animals — known as Ichthyostega — was also found to have an assortment of hitherto unknown skeletal features including a string of bones extending down the middle of its chest.
"These chest bones turned out to be the earliest evolutionary attempt to produce a bony sternum," said Clack. “Such a structure would have strengthened the ribcage of Ichthyostega, permitting it to support its body weight on its chest while moving about on land."
This unexpected discovery supports recent work done by the same authors that showed Ichthyostega probably moved by dragging itself across flat ground using synchronous crutching motions of its front legs — much like that of a mudskipper or seal.
The next step, the researchers say, is to understand how the backbone aided locomotion in these early tetrapods using sophisticated biomechanical analysis.
For further information see Backbone.
Terapod image via University of Cambridge.