I found a Net. Geo article about the limbed fish Tiktaalik with some more info, including a possible explanation on its huge head. Didn't someone bring that up? Anyway, apparently the story is that it hunted like a crocodile, and so it needed large jaws. Also, becuase it had nostrils, it need to breathe through its head too. It also seemed to move like modern mudskippers in shallow flooded land. Here's the link:
http://news.nationalgeographic.com/news/2006/04/0405_060405_fish.html