Identity of the avian wing digits: Problems resolved and unsolved


  • In keeping with the traditional nomenclature, Arabic numerals (1–5) will be used when referring to the embryological position of a digit in the developing autopod regardless of the digit's terminal identity. Roman numerals (I–V) will be used when referring to digit identity.


Controversy over bird wing digit identity has been a touchstone for various ideas in the phylogeny of birds, homology, and developmental evolution. This review summarizes the past 10 years of progress toward understanding avian digit identity. We conclude that the sum of evidence supports the Frame Shift Hypothesis, indicating that the avian wing digits have changed anatomical location. Briefly, the derivation of birds from theropod dinosaurs and the positional identities of the avian wing digits as 2, 3, and 41 are no longer in question. Additionally, increasing evidence indicates that the developmental programs for identity of the wing digits are of digits I, II, and III. Therefore, the attention moves from whether the digit identity frame shift occurred, to what the mechanisms of the frame shift were, and when in evolution it happened. There is considerable uncertainty about these issues and we identify exciting new research directions to resolve them. Developmental Dynamics 240:1042–1053, 2011. © 2011 Wiley-Liss, Inc.