Acanthostega
"This animal, though clearly a tetrapod, was primarily an aquatic creature whose immediate forerunners were essentially fish that never left the water. The discovery forced scholars to rethink the sequence in which key changes to the skeleton took place. Rather than portraying a creature like Eusthenopteron crawling onto land and then gaining legs and feet, as Romer postulated, the new fossils indicated that tetrapods evolved these features while they were still aquatic and only later co-opted them for walking".