FROM FROND TO FAN - ARCHAEOPTERYX AND THE EVOLUTION OF SHORT-TAILED BIRDS

Modern birds have extremely short tail skeletons relative to Archaeopt eryx and nonavialian theropod dinosaurs. Long- and short-tailed birds also differ in the conformation of main tail feathers making up the fl ight surface: frond shaped in Archaeopteryx and fan shaped in extant f liers. Mechanisms of tail fanning were evaluated by electromyography i n freely flying pigeons and turkeys and by electrical stimulation of c audal muscles in anesthetized birds. Results from these experiments re veal that the pygostyle, rectrices, rectricial bulbs, and bulbi rectri cium musculature form a specialized fanning mechanism. Contrary to pre vious models, our data support the interpretation that the bulbi rectr icium independently controls tail fanning; other muscles are neither c apable of nor necessary for significant rectricial abduction. This bul b mechanism permits rapid changes in tail span, thereby allowing the e xploitation of a wide range of lift forces. Isolation of the bulbs on the pygostyle effectively decouples tail fanning from fan movement, wh ich is governed by the remaining caudal muscles. The tail of Archaeopt eryx, however, differs from this arrangement in several important resp ects. Archaeopteryx probably had a limited range of lift forces and ti ght coupling between vertebral and rectricial movement. This would hav e made the tail of this primitive flier better suited to stabilization than maneuverability. The capacity to significantly alter lift and ma nipulate the flight surface without distortion may have been two facto rs favoring tail shortening and pygostyle development during avian evo lution.