Research Into Locomotion And The First Amphibians

The origins of the amphibians is one of the most fascinating topics in evolutionary biology and palaeontology. At one point there were few fossils in the geological record to work from which led to the creation of Alfred Romer's Romer's Gap, but recently new discoveries reveal how events progressed from scale-covered, water-dwelling fish to slimy, air-gulping amphibians.

Palaeontologists have been more focused on why vertebrates left the water.

The vertebrae of Ichthyostega based on the synchrotron data

Yet recently new analysis of fossils which were discovered over 50 years ago have thrown up surprises. Specimens of one of the first amphibians, a creature called Ichthyostega, were taken to the European Synchrotron Radiation Facility, a particle accelerator which produces x-rays so powerful that they can penetrate solid rock. Over the years, many highly significant fossil-based discoveries have been made at the Swiss Light Source.

This time, data from the synchrotron revealed a surprising feature of the backbones of early amphibians. The vertebrae of modern tetrapods are composed of single pieces of bone, but in early forms, namely the first amphibians, they were built from four separate bones: one on the top and one below, connected to another pair. The synchrotron models showed that this idea was, in fact, back-to-front.

The bone known as the intercentrum was the last in the series rather than the first. While this may seem unimportant, it has implications for how the creature may have moved. '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,' said Stephanie Pierce, the lead author of the synchrotron study.

Seals demonstrating the flexibility of their spines.

Another surprising discovery was a chain of bones extending down the length of the chest; the first prototype of a sternum. 'Such a structure would have strengthened the rib cage of Ichthyostega, permitting it to support its body weight on its chest while moving about on land,' said Professor Jenny Clack, who is known for her work on early tetrapods.

This new, weight-bearing sternum combined with a highly flexible spine suggests that it may have moved rather like a seal or a mud skipper. By arching its spine, pushing its torso forwards and pulling itself along with its front legs, it used a form of locomotion properly called crutching. Indeed, the limbs of the first amphibians would have been weak and so crutching would have utilized the power of the core muscles. These would have been far more developed considering that they evolved from fish which rely on core muscles to power their tails in order to swim.

The story of amphibians has become clearer in recent years. Yet it was only around 25 years ago that fossils of the earliest amphibians became known to the general scientific community and the evolution of the first amphibians was reconstructed. Since then we now know what they first looked like, where they lived and why they evolved. And like good whiskey or wine, with time our understanding of their history becomes richer and more complex, not to mention enjoyable.