Dragons evolved from tree-dwelling gliders similar to Pterosaurs, and mostly lived on insects and small rodents until the late Triassic. Climate changes as the continents began to drift eventually drove them out of the trees, and into the water, where their diet shifted to fish. For approximately five million years they adapted to life much like modern water-fowl, burying their eggs in the mud and ambushing their prey from above. Later, as their wings became increasingly used more for swimming than flight, they would become giant water-dwelling behemoths capable of holding their breaths for an hour at a time, only leaving the water once every five years to lay eggs on the same mud-banks they were hatched.
Today, all that remains are their bones, and the rare sighting of a mysterious long-necked reptile rising from the depths, breaking the water's surface to breathe.
EDIT: Thank you for the DD, everyone!