The Feathers of the Jurassic Urvogel Archaeopteryx

Abstract

The Jurassic stem bird Archaeopteryx is an iconic transitional fossil, with an intermediate morphology combining features of non-avian dinosaurs and crown Aves. Importantly, fossils of Archaeopteryx preserve not only the bones but also details of the plumage and therefore help shed light on the evolution of feathers, wings, and avian flight. Plumage is preserved in multiple individuals, allowing a detailed documentation of the feathers of the wings, tail, hindlimbs, and body. In some features, Archaeopteryx' plumage is remarkably modern, yet in others, it is strikingly primitive. As in extant birds, remiges and coverts are enlarged and overlap to form airfoils. Remiges and rectrices exhibit asymmetrical, pennaceous vanes, with interlocking barbules. The hindlimbs bear large, vaned feathers as in Microraptor and Anchiornis. Rectrices are numerous and extend the full length of the tail to the hips. The plumage of crown Aves was assembled in a stepwise fashion from Anchiornis through Archaeopteryx, culminating in a modern arrangement in ornithothoracines. Subsequent stasis in feather and wing morphology likely reflects aerodynamic and developmental constraints. Feather morphology and arrangement in Archaeopteryx are consistent with lift-generating function, and the wing loading and aspect ratio are comparable to modern birds, consistent with gliding and perhaps flapping flight. The plumage of Archaeopteryx is intermediate between Anchiornis and more derived Pygostylia, suggesting a degree of flight ability intermediate between the two.