feathers

evolution of feathers
left: diagram showing possible early stages in the evolution of feathers (adapted from the work of Prum, 1999). The precursor of a feather may have been a conical papilla developed from a cylindrical follicle within the skin. In the next stage, the papilla becomes a tuft of unbranched filaments (barbs), and then each filament becomes branched (barbules). At some point the branched filaments appear to have become organised around a central stem (rachis) to produce the pennate arrangement of present-day feathers. Traditionally, the evolution of feathers has been linked to the origin of flight. However, there is increasing evidence that true feathers emerged in small non-flying dinosaurs at an earlier time, perhaps as a means of increasing insulation for warm-blooded species (Norell et al, 2002). Xu, Zhou, and Prum (2001) describe filamentous skin structures in fossils of Sinornithosaurus millenii, a non-avian theropod dinosaur that lived about 125 million years ago. The fossils were found in north-eastern China. The filamentous structures show two characteristics that are otherwise considered to be unique to bird feathers: filaments joined in a basal tuft, and filaments joined in series along a central filament. They resemble contour feathers, and no flight feathers are apparent. In another find at the same location, a nearly complete specimen has been found of a small theropod dinosaur, the body, limbs and tail of which is densely covered with feather-like structures (Ji et al, 2001). The fossils of Archaeopteryx show feathers that have a similar structure to present-day avian feathers.

development of feathers
	Feathers are a distinctive feature of present-day birds, and contribute significantly to control of body temperature, flight, and swimming. They are characterised by a complex branched structure built from filaments of keratin, and develop from cylindrical feather follicles. There are three levels of branching: from rachis to barbs, from barbs to barbules, and from barbules to hooklets. Yu et al (2002) have identified some of the molecular pathways that contribute to the transformation of cylindrical feather primordia into hierarchically-branched feathers during development, and have related these to the currently limited evidence for the evolution of feathers.
left: diagram of a developing feather. In this follicle, a larger main feather and a smaller afterfeather are developing.
References Ji, Q., Norell, M.A., Gao, K-Q., Ji, S-A., and Ren, D. (2001) The distribution of integumentary structures in a feathered dinosaur. Nature, 410, 1084-1088 (April 26). Norell, M., Ji, Q., Gao, K., Yuan, C., Zhao, Y., and Wang, L. (2002) 'Modern' feathers on a non-avian dinosaur. Nature, 416, 36-37 (Mar 7). Prum, R.O. (1999) Development and evolutionary origin of feathers. Journal of Experimental Zoology (Molecular and Developmental Evolution), 285(4), 291-306. Sues, H-D. (2001) Ruffling feathers. Nature, 410, 1036-1037 (Apr 26). Xu, X., Zhou, Z-h., and Prum, R.O. (2001) Branched integumental structures in Sinornithosaurus and the origin of feathers. Nature, 410, 200-204 (Mar 8). Yu, M., Wu, P., Widelitz, R.B., and Chuong, C-H. (2002) The morphogenesis of feathers. Nature, 420, 308-312 (Nov 21).