The realization that dinosaurs are closely related to birds raised the obvious possibility that some dinosaurs had feathers.
Fossils of Archaeopteryx include well-preserved feathers, but it was not until the early 1990’s that clearly nonavian dinosaur fossils were discovered with preserved feathers.
Today there are more than a dozen genera of dinosaurs with fossil feathers, all of which are theropods.
Most are from the Yixian formation in China.
The fossil feathers of one specimen, Shuvuuia deserti, have even tested positive for beta keratin, the main protein in bird feathers, in immunological tests.
Particularly well-preserved (and legitimate) fossils of feathered dinosaurs were discovered during the 1990s and 2000s.
The fossils were preserved in a Lagerstätte -- a sedimentary deposit exhibiting remarkable richness and completeness in its fossils -- in Liaoning, China.
The area had repeatedly been smothered in volcanic ash produced by eruptions in Inner Mongolia 124 million years ago, during the Early Cretaceous Period.
The fine-grained ash preserved the living organisms that it buried in fine detail.
The area was teeming with life, with millions of leaves and the oldest known angiosperms, insects, fish, frogs, salamanders, mammals, turtles, lizards and crocodilians discovered to date.
The most important discoveries at Liaoning have been a host of feathered dinosaur fossils, with a steady stream of new finds filling in the picture of the dinosaur-bird connection and adding more to theories of the evolutionary development of feathers and flight.