Between water and land

Abstract

The most informative examples of large-scale evolution are provided by major transitions between environments. Fresh research on an ancient amphibian shows how it adapted to locomotion both in water and on land. The origin of tetrapods during the Devonian period, around 350 million years ago, was a key step in vertebrate evolution. The first and arguably the single most important discovery in the study of Devonian tetrapods was Ichthyostega, the four-legged fish from Greenland first described in 1932. Now, based on the original material and more recently collected specimens, a new reconstruction of Ichthyostega has been made. It differs radically from previous versions in having a regionalized vertebral column that bears a striking resemblance to that of a mammal. The presacral vertebral column appears to have almost no lateral flexibility, but there is vertical flexibility in the lumbar region. This suggests that Ichthyostega could move on land using a bilaterally symmetrical ‘shuffling’ action. It may have been an early and ultimately unsuccessful attempt at adapting the tetrapod body plan of terrestrial locomotion, a problem solved by a tetrapod lineage quite closely related to Ichthyostega.