Losing Your Tail

Sapeornis was one of the first primitive birds lacking a bony tail. Ancestors to this and some other primitive birds still possessed bony elongate tails. The newer crop of birds had evolved shorter, more derived (and "modern") tails consisting of a pygostyle and retrices attached to the this feature. The pygostyle of modern birds is composed of variably curved fused caudal vertebrae on which specialized tail feathers, called retrices, attach. These tail feathers can be stiff and supportive like in woodpeckers, long and showy as in some hummingbirds, or agile and utilitarian like those seen in hawks and other soaring birds. There is a spectrum of other varieties within this diversity as well. Sapeornis possessed an intermediate pygostyle that was still longer than the modern equivalents but was very rod-like. As of now there is no evidence of the tail feathers, however they undoubtedly existed in some form, as seen in this illustration:

©Yike Xu