I've had the priviledge of many a late night call recently with Xu Xing, a paleontologist at the Chinese Academy of Sciences in Beijing who has described more dinosaurs than anyone else alive.
Xu is a leading proponent of the theory that birds evolved from dinosaurs and over the past decade has uncovered a number of really unusual fossils, from a pint sized dinosaur with four wings to a feathered ancestor of Tyrannosaurus rex. You can read my interview with Xu in New Scientist, and find additional illustrations of what these feathered beasts likely looked like here.
Having once worked as a biologist reintroducing California condors to the wild, the theory that birds evolved from dinosaurs seemed obvious to me. Condors have hard scales on their feet that, when seen up close, seem to scream out "we stem from an ancient line of reptiles!"
Talking with Xu, however, it seems my observations were only partly correct. Condors and all other birds do descend from dinosaurs, but the commonly held belief that scales = ancient and feathers = modern isn't so simple.
As Xu describes it, feathers likely first appeared in dinosaurs as little more than eye candy for perspective mates. As a result, they weren't an essential part of dino anatomy and their existence tended to drift in and out of different lineages depending on what was fashionable at the time.
A good example of this is guanlong, a feathered ancestor of T Rex that lived 160 million years ago. Nearly 100 million years later, guanlong's ancestor, T Rex, walked the earth with nary a feather on its body. So based on what Xu tells me, it's the feathers as much as the scales on a modern condor that reveal the bird's ancient lineage.