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ARISING FROM Xing Xu, Xiaoting Zheng & Hailu You Nature 464, 1338-1341 (2010)
Xu et al.1 describe the extraordinarily preserved feathers from two subadults of the oviraptorisaur Similicaudipteryx from the Yixian Formation of Liaoning, China. The preserved tail feathers of the juvenile specimen (STM4.1) show a morphology not previously observed in any fossil feathers. The tail feathers of an older, immature specimen (STM22-6) show a typical closed pennaceous structure with a prominent, planar vane. I propose that the feathers of the tail of the juvenile specimen are not a specialized feather generation, but fossilized 'pin feathers' or developing feather germs.
Xu et al.1 interpret the juvenile Similicaudipteryx tail feathers as examples of ''proximately ribbon-like pennaceous feathers'' (PRPFs) that have evolved convergently in avialian confuciusornithids and enanthiornithines, and in the non-avian maniraptoran Epidexipteryx. They describe the differences between juvenile and immature Similicaudipteryx feathers as a notable example of post-nestling ontogenetic change in feather morphology, and claim that ''this phenomenon is not known to occur in other birds.''
Althoughmodern birds do not show radical changes in flight-feather morphology after the nestling stage (probably owing to the functional constraints of flight), there are many examples of radical post-nestling changes in the morphology of other feathers. For example, the feather follicles on the head of a Wild Turkey (Meleagris gallopavo) grow plumulaceous down in nestlings, fully pennaceous contour feathers in juveniles, and specialized bristles in adults2.
Data from Xu et al.1 document that the tail feathers of the juvenile Similicaudipteryx specimen are very different in morphology from previously described PRPFs, in which the ribbon-like basal portion is formed completely by the lateral expansion and vertical compression of the rachis (Supplementary Fig. 3 in Xu et al.1). In contrast, the unbranched, proximal portion of the tail feathers of the juvenile Similicaudipteryx is not a merely a continuous, basal extension of the rachis (Supplementary Fig. 2a inXuet al.1). Rather, the undifferentiated basal portion of these feathers surrounds the entire base of the exposed distal vane of the feather. The distal rachis does not expand laterally, and is visibly distinct from the broad base of the feather (top of Supplementary Fig. 2a in Xu et al.1). This observation is inconsistent with the conclusion of Xu et al.1
The juvenile tail feathers of...