By studying the limb bones of early land animals, British and Swedish researchers have discovered some differences that could modify what we know about the first four-legged land creatures that crawled out of the sea to live on land. As revealed in the journal Science by Vivianne Callie of the Cambridge Museum of Zoology and her colleagues, the position of the upper arm bones of the Ichthyostega slowly changed as it aged. This finding suggests that Ichthyostega spent more time in the water when he was young and then moved on land, making him the most aquatic tetrapod known.
Until now researchers believed that Acanthostega was the most fish-shaped land animal in the fossil record, but the discovery of the migration of the forelimbs of Ichthyostega indicates that it should occupy a deeper position in the evolutionary tree. In fact, Callie concludes that we are looking at the most primitive fossil evidence of a transition between aquatic and terrestrial environments .
