Most scientists believe that birds descended from theropod dinosaurs. But
theropod dinosaurs have lost digits IV and V from a primitively
five-fingered hand, leaving them with digits I, II, and III. Birds, on
the other hand, have digits II, III, and IV, having lost digits I and V
(Feduccia and Nowicki 2002). It is almost impossible for the two groups
to be closely related with such a significant anatomical difference.
Anatomists initially thought bird digits were I, II, II based on their
anatomy. This was revised on the basis of bird embryology; the digits
are seen to derive from condensations II, III, IV. It is plausible
that dinosaur digits also developed from condensations II, III, IV, and
a frame shift in the development of digit identity causes those three
condensations to developed into digits I, II, III. Such a frameshift
occurs in kiwis, in which digits II, III take the form of I, II with
the loss of the condensation for digit I (Wagner and Gauthier 1999).
Feduccia, A. and J. Nowicki. 2002. The hand of birds revealed by early
ostrich embryos. Naturwissenschaften 89: 391-393.
Wagner, G. P. and J. A. Gauthier. 1999. 1,2,3=2,3,4: A solution to the
problem of the homology of the digits in the avian hand. Proceedings of
the National Academy of Science USA 96:
5111-5116.
Further Reading:
Pennisi, Elizabeth. 2005. Bird wings really are like dinosaurs' hands.
Science 307: 194-195.
