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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. 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. |
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Feathers are a distinctive feature of present-day birds. They are characterised by a complex branched structure built from filaments of keratin, and develop from cylindrical feather follicles. | ||
 left:
diagram of a developing feather. In this follicle, a larger main feather
and a smaller afterfeather are developing. |
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| 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). 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).
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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.