Recent work shows that basal dromaeosaurs closely resemble Archaeopteryx in flight apparatus7, 8, 20, 27 (except for some differences in limb proportion, such as the longer arm in the latter taxon), and thus dromaeosaurs were pre-adapted for flight. The new discoveries provide further information on soft tissues and greatly improve our knowledge of these close relatives of birds. Asymmetrical pennaceous feathers are suggested to have the aerodynamic function necessary for flight28. The asymmetry is present not only on the forelimb and tail feathers, but also the hindlimb feathers of basal dromaeosaurs (Fig. 3c–f). The forelimb and the leg feathers would make a perfect aerofoil together, analogous to the patagium in bats or gliding animals. These features together suggest that basal dromaeosaurids probably could glide, representing an intermediate stage between the flightless non-avian theropods and the volant avialans. Apparently some non-avian theropods evolved large and highly specialized pennaceous feathers on the leg for aerodynamic function; these features were later reduced and lost in birds, which depend completely on forewings for flight.
The metatarsus feathers are inconsistent with the suggestions that basal dromaeosaurs are cursorial animals1, 29 because such long feathers on the feet would be a hindrance for a small cursorial animal. It is unlikely that a small dromaeosaur could run fast with such an unusual integument and this provides negative evidence for the ground-up hypothesis for the origin of avian flight29-31. Some recent osteological studies suggested that non-avian theropods8, 20, 32 and basal birds33-36 acquired arboreal capabilities, which were later improved in more derived birds20, 37, 38. Combined with the new information from the integument, we suggest that basal dromaeosaurs were arboreal animals, and that the ancestor of birds first learned to glide by taking advantage of gravity before flapping flight was acquired in birds32, 39-44. |
