Size and Weight:
Archaeopteryx is so firmly rooted in the popular imagination as the "first bird" that any feathered dinosaur that precedes it in the fossil record is bound to cause a sensation. Witness the case of Epidexipteryx, which predated Archaeopteryx by as much as 15 million years (the sediments in which the "type fossil" was found make more precise dating impossible). The most striking feature of this tiny "dino-bird" was the spray of feathers shooting out from its tail, which clearly had an ornamental function. The rest of this creature's body was covered with much shorter, more primitive plumes that may (or may not) have represented an early stage in the evolution of true feathers.
Was Epidexipteryx a bird or a dinosaur? Most paleontologists hew to the latter theory, classifying Epidexipteryx as a small theropod dinosaur closely related to the equally tiny Scansoriopteryx (which lived at least 20 million years later, during the early Cretaceous period). However, one rogue theory proposes that not only was Epidexipteryx a true bird, but that it had "de-evolved" from flying birds that lived millions of years earlier, during the early Jurassic period. This seems unlikely, but the discovery of Epidexipteryx does raise the question of whether feathers evolved primarily for flight, or began as a strictly ornamental adaptation meant to attract to opposite sex.