Been meaning to link this one up, but kept forgetting: scientists in China have discovered a large group of well-preserved fossil birds from the middle Cretaceous period. When combined with earlier bird finds from a different branch of the family tree, it appears to imply that birds split into two main types very early on, one adapted to land, the other to water. Later, perhaps due to the K-T impact, the land branch died out and the water branch moved back to land.