A Morphological Review of the Enigmatic Elongated Tail Feathers of Stem Birds

Abstract

Several stem birds, such as Confuciusornithidae and Enantiornithes, were characterized by the possession of one or two pairs of conspicuous, elongated tail feathers with a unique morphology, so-called rhachis-dominated racket plumes. In the past, several studies reported contradictory interpretations regarding the morphology of these feathers, which sometimes failed to match with any morphology known from modern feathers. In this chapter, these interpretations are reviewed and compared with various modern feather types. The comparison confirms recent interpretations that the rhachis-dominated racket plumes are highly modified pennaceous feathers with ornamental function, originating at least two times independently from each other during evolution. While the gross organization (i.e., a short distal vane and a long, naked rhachis) of these feathers resembles that of filoplumes, they resemble pennaceous body feathers of penguins in terms of rhachis morphology and pigmentation pattern. As the rhachis-dominated racket plumes combine different morphologies that are apparent among modern feather types, this extinct morphotype does in fact not show any aberrant morphological novelties, but rather fall into the morphological and developmental spectrum of modern feathers.