Five into Three Is . . .

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Science  06 Sep 2002:
Vol. 297, Issue 5587, pp. 1611
DOI: 10.1126/science.297.5587.1611d

The identity of the digits of the avian hand has been hotly debated since the 1820s. Early in development, birds display four cartilaginous digits, with the distal digit (V) a rudimentary splint; however, mature birds possess only three ossified fingers. Studies of fossils and analyses of phylogenies suggest that the mature fingers are equivalent to digits I, II, and III of pentadactyl amniotes, but embryological evidence favors a homology of II, III, and IV. Two recent studies offer new results that bear on this question. Larsson and Wagner used molecular markers to identify digit condensations of chicken embryos, and Feduccia and Nowicki examined the digit anlagen of ostrich embryos. Both studies identify a single digit anlage that is in a proximal position relative to the three ossified digits. Hence, bird fingers develop from digit anlagen II, III, and IV. The findings have implications for the evolutionary relationship between birds and theropod dinosaurs, which display a I, II, III digit identity.— BAP & ShJS

J. Exp. Zool.294, 146 (2002); Naturwissenschaften 10.1007/s00114-002-0350-y (2002).

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