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The first evidence for feathers in tyrannosauroids came from the small species Dilong paradoxus, found in the Yixian Formation of China, and reported in 2004. As with many other theropods discovered in the Yixian, the fossil skeleton was preserved with a coat of filamentous structures which are commonly recognized as the precursors of feathers.[35] Because all known skin impressions from larger tyrannosauroids known at the time showed evidence of scales, the researchers who studied Dilong speculated that feathers may correlate negatively with body size—that juveniles may have been feathered, then shed the feathers and expressed only scales as the animal became larger and no longer needed insulation to stay warm.[35] However, subsequent discoveries showed that even some large tyrannosauroids had feathers covering much of their bodies, casting doubt on the hypothesis that they were a size-related feature.[36]