Feathered Dinosaurs and the Origin of Birds
In recent years, some of the most significant specimens for understanding evolution from dinosaurs to birds have been recovered from the Lower Cretaceous rocks (120-145 Ma) of northeastern China. These include more than 1000 specimens of the primitive bird Confuciusornis, and more than 20 skeletons of at least 6 species of dinosaurs with feathers.
Sinosauropteryx, the first feathered dinosaur to be recovered, was recovered in 1996. It is closely related to Compsognathus, one of the smallest dinosaurs known. However, it had a longer tail and shorter arms, and downy feathers insulated its body. Its discovery sparked a heated debate on the origin of birds. However, the following year another feathered dinosaur was discovered. In addition to the downy body covering, Caudipteryx has well-developed feathers on the arms and tail. The structure of the feathers and taxonomic position of this animal swayed most scientists to now accept that birds are the direct descendents of meat-eating dinosaurs.
Feathers were probably widely distributed among meateating dinosaurs, and we can no longer be sure that fossilized feathers found in Cretaceous rocks all belong to birds. More recent discoveries include a man-sized feathered therizinosaur known as Beipiaosaurus, and a feathered dromaeosaur called Sinornithosaurus. A specimen of an oviraptorosaur from Mongolia has a pygostyle at the end of the tail, which also suggests that these dinosaurs had feathers.
The two distinct types of feathers associated with these animals suggest that feathers evolved from a simple branching structure that became progressively more complex, and that they were initially used for insulation and display. The evidence favors the theory that flight evolved from the ground up. Regardless of the widespread presence of feathers in dinosaurs, the theory that birds evolved from dinosaurs is supported by more than 125 osteological characters that are uniquely shared by these two groups of animals.
Although no dinosaur specimens with preserved feathers have been found in North America, many of the Late Cretaceous species from Alberta and other regions are closely related to the feathered dinosaurs of China. It is highly likely that most of the Late Cretaceous theropods of the Northern Hemisphere, including tyrannosaurs, were feathered.