An ongoing debate in the paleontological community surrounds the extent and nature of tyrannosaurid integumentary covering. Long filamentous structures have been preserved along with skeletal remains of numerous coelurosaurs from the Early Cretaceous Yixian Formation and other nearby geological formations from Liaoning, China. These filaments have usually been interpreted as "protofeathers," homologous with the branched feathers found in birds and some non-avian theropods, although other hypotheses have been proposed. A skeleton of "Dilong" was described in 2004 that included the first example of "protofeathers" in a tyrannosauroid. Similarly to down feathers of modern birds, the "protofeathers" found in "Dilong" were branched but not pennaceous, and may have been used for insulation. The discovery and description of the feathered tyrannosauroid "Yutyrannus" in 2012 indicates the possibility large tyrannosaurids were also feathered as adults.