Primitive Wing Feather Arrangement in Archaeopteryx lithographica and Anchiornis huxleyi

In modern birds (Neornithes), the wing is composed of
a layer of long, asymmetrical flight feathers overlain by short
covert feathers [1–3]. It has generally been assumed that
wing feathers in the Jurassic bird Archaeopteryx [4–9] and
Cretaceous feathered dinosaurs [10, 11] had the same
arrangement. Here, we redescribe the wings of the archaic
bird Archaeopteryx lithographica [3–5] and the dinosaur
Anchiornis huxleyi [12, 13] and show that their wings differ
from those of Neornithes in being composed of multiple
layers of feathers. In Archaeopteryx, primaries are overlapped
by long dorsal and ventral coverts. Anchiornis has
a similar configuration but is more primitive in having short,
slender, symmetrical remiges. Archaeopteryx and Anchiornis
therefore appear to represent early experiments in the
evolution of the wing. This primitive configuration has
important functional implications: although the slender
feather shafts of Archaeopteryx [14] and Anchiornis [12]
make individual feathers weak, layering of the wing feathers
may have produced a strong airfoil. Furthermore, the layered
arrangement may have prevented the feathers from forming
a slotted tip or separating to reduce drag on the upstroke.
The wings of early birds therefore may have lacked the range
of functions seen in Neornithes, limiting their flight ability.