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Embryological evidence that bird fingers are I, II, III

Matthew Towers, Jason Signolet, Adrian Sherman, Helen Sang & Cheryll Tickle (9 August 2011): Insights into bird wing evolution and digit specification from polarizing region fate maps, Nature Communications 2, article number 426

Abstract and figures here: http://www.nature.com/ncomms/journal/v2/n8/full/ncomms1437.html


The proposal that birds descended from theropod dinosaurs with digits 2, 3 and 4 was recently given support by short-term fate maps, suggesting that the chick wing polarizing region -- a group that [sic] Sonic hedgehog-expressing cells -- gives rise to digit 4. Here we show using long-term fate maps that Green fluorescent protein-expressing chick wing polarizing region grafts contribute only to soft tissues along the posterior margin of digit 4, supporting fossil data that birds descended from theropods that had digits 1, 2 and 3. In contrast, digit IV of the chick leg with four digits (I--IV) arises from the polarizing region. To determine how digit identity is specified over time, we inhibited Sonic hedgehog signalling. Fate maps show that polarizing region and adjacent cells are specified in parallel through a series of anterior to posterior digit fates -- a process of digit specification that we suggest is involved in patterning all vertebrate limbs with more than three digits.


Anterior to posterior? I can't wait to read the paper and ponder its implications for salamanders, temnospondyls, anthracosaurs, colosteids... :-)