Feathers are a familiar trait associated with birds. Until recently, the presence of feather impressions in a fossil meant that a specimen was a bird. This assumption held true for Archaeopteryx in 1861 and remained unquestioned until the description of Sinosauropteryx in 1996. The skin of Sinosauropteryx, a non-avian dinosaur, was covered with protofeathers, but because that animal clearly could not fly, paleontologists had to entertain the possibility that the
evolution of feathers and flight were not as intimately connected as had once been believed. Subsequent to the discovery of Sinosauropteryx, a host of discoveries of non-avian theropods, including those of Sinornithosaurus and Caudipteryx, showed irrefutable evidence that some small, flightless dinosaurs were equipped with feathers. By decoupling the presence of feathers from the ability to fly, it was possible to consider feathers as an inherited trait that birds acquired from non-avian dinosaur ancestors. It is now assumed that ancestral coelurosaurians had feathers before the evolution of birds.
The first feathers probably insulated small theropods from temperature extremes, allowing them to regulate their body temperature more effectively. Gradually, as the arms of some small coelurosaurs evolved into more winglike forelimbs, feathers that once were used solely for insulation acquired the secondary function of creating a feathered covering on the forelimb. These feathers served as an airfoil that enabled some theropods to glide and, later, to engage in powered flight.
Feathers are such unique, complex structures that they probably evolved only once. Birds and dinosaurs are the only creatures known to have had feathers. Unless other fossil evidence is found that might prove otherwise, the presence of feathers is one of the most compelling reasons to believe that birds evolved from small, feathered dinosaurs.
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