Acanthostega is seen as part of widespread evolutionary radiation in the late Devonian period , starting with purely aquatic finned tetrapodomorphs , with their successors showing increased air-breathing capability and related adaptions to the jaws and gills , as well as more muscular neck allowing freer movement of the head than fish have , and use of the fins to raise the body of the fish .
They had a more muscular neck allowing freer movement of the head than fish have , and used their fins to raise the body of the fish .