because of aquatic or semiaquatic adaptations. Body integument 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 feathers in a tyrannosauroid. Similarly to down feathers of modern birds, the feathers found in Dilong were branched but not pennaceous, and may have been used for insulation. Even large tyrannosauroids have been found with evidence of feathers. Yutyrannus huali, also from the Yixian Formation, is known from three specimens, each preserving traces of feathers on various parts of the body. While not all areas of the body preserve impressions across all three specimens, these fossils demonstrate that even in this medium-sized species, most of the body was covered in feathers. The presence of feathers in basal tyrannosauroids is not surprising since they are now known to be characteristic of coelurosaurs, found in other basal genera like Sinosauropteryx, as well as all more derived groups. Rare fossilized skin impressions of some Late Cretaceous tyrannosaurids lack feathers, however, instead showing skin covered in fine, non-overlapping scales. Possibly, feathers were present on other areas of the body: Preserved skin impressions are very small and come primarily from the legs, pelvic region, and underside of the tail, which either lack feathers or only covered in a light down in some modern larger ground-dwelling birds. Alternatively, secondary loss of feathers in large tyrannosaurids may be analogous with the similar loss of hair in the largest modern mammals like elephants, where a low surface area-to-volume ratio slows down heat transfer, making insulation by a coat of hair unnecessary or even detrimental. A scientific publication by Phil Bell and colleagues in 2017 show that tyrannosaurids such as Gorgosaurus, Tarbosaurus, Albertosaurus, Daspletosaurus, and Tyrannosaurus had scales. The Bell et al. 2017 paper notes that the scale-like integument on bird feet were actually secondarily derived feathers according to paleontological and evolutionary developmental evidence so they hypothesize that the scaly skin preserved on some tyrannosaurid specimens might be secondarily derived from filamentous appendages like on Yutyrannus although strong evidence is needed to support this hypothesis. However, other paleontologists argue that taphonomy is the possible