As I discuss in WEIT, the evidence shows that birds evolved from theropod dinosaurs–gracile, carnivorous beasts that walked on two legs. Some of the important evidence comes from Chinese fossils showing theropods with various types of feathers. The incipient stages of feather evolution appears to be filamentous feathers (T. rex might well have been covered with fluff!), implying that flight feathers originated as devices to help insulate the theropods. In a recent paper in Nature, however, F. Zhang et al. found a theropod fossil (Epidexipteryx hui), about 160 million years old, which had not only the downy feathers (feathers completely unsuitable for flight) but also four very long tail feathers that could not have been used for either flight or insulation. E. hui also showed a number of morphological features that seem to make it closely related to modern birds. The most likely explanation for these tail feathers is that they were ornaments–ornaments that evolved for either species recognition or via sexual selection. The evolution of flight, then, may have begun with feathers that were used for display. A somewhat fanciful reconstruction of the beast is shown below.
