From frond to fan: Archaeopteryx and the evolution of short-tailed birds

Modern birds have extremely short tail skeletons relative to Archaeopteryx and nonavialian theropod dinosaurs. Long- and short-tailed birds also differ in the conformation of main tail feathers making up the flight surface: frond shaped in Archaeopteryx and fan shaped in extant fliers. Mechanisms of tail fanning were evaluated by electromyography in freely flying pigeons and turkeys and by electrical stimulation of caudal muscles in anesthetized birds. Results from these experiments reveal that the pygostyle, rectrices, rectricial bulbs, and bulbi rectricium musculature form a specialized fanning mechanism. Contrary to previous models, our data support the interpretation that the bulbi rectricium independently controls tail fanning; other muscles are neither capable of nor necessary for significant rectricial abduction. This bulb mechanism permits rapid changes in tail span, thereby allowing the exploitation of a wide range of lift forces. Isolation of the bulbs on the pygostyle effectively decouples tail fanning from fan movement, which is governed by the remaining caudal muscles. The tail of Archaeopteryx, however, differs from this arrangement in several important respects. Archaeopteryx probably had a limited range of lift forces and tight coupling between vertebral and rectricial movement. This would have made the tail of this primitive flier better suited to stabilization than maneuverability. The capacity to significantly alter lift and manipulate the flight surface without distortion may have been two factors favoring tail shortening and pygostyle development during avian evolution.