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New bird hand digit papers

From: Ben Creisler

New papers about digits in bird hand:

Gabe S. Bever, Jacques A. Gauthier, and Günter P. Wagner (2011)
Finding the frame shift: digit loss, developmental variability, and the
origin of the avian hand
Evolution & Development 13(3): 269?279, May/June 2011
Article first published online: 3 MAY 2011
DOI: 10.1111/j.1525-142X.2011.00478.x

The origin of the tridactyl hand of crown birds from the pentadactyl hand
of those early theropod dinosaurs lying along the avian stem has become a
classic, but at times seemingly intractable, historical problem. The point
in question is whether the fingers of crown birds represent digits 1?3 as
predicted by generalized trends in the fossil record; or digits 2?4, as
evidenced by the topology of the embryonic mesenchymal condensations from
which the digits develop. The frame shift hypothesis attempted to resolve
this paradox by making these signals congruent by means of a homeotic
transformation in digital identity, but recently the paleontological
support for this hypothesis was questioned. Here, we reassess the frame
shift from a paleontological perspective by addressing what predictions a
frame shift makes for skeletal morphology, whether the frame shift remains
a viable explanation of the known fossil data, and where on the theropod
tree the frame shift most likely occurred. Our results indicate that the
frame shift remains viable, and based on the inferred pattern of digit
loss, the frame shift likely occurred at a deeper position in theropod
phylogeny than previously proposed. A new evolutionary model of the frame
shift is described in which the early history of the frame-shifted hand is
marked by an extended zone of developmental polymorphism. This model
provides a new conceptual framework for the role of developmental
variability in communicating broad evolutionary patterns on a taxonomically
inclusive phylogenetic tree.

Rebecca L. Young, Gabe S. Bever, Zhe Wang, and Günter P. Wagner (2011)
Identity of the avian wing digits: Problems resolved and unsolved.
Developmental Dynamics Special Issue: Special Issue on Limb Development
240(5): 1042?1053, May 2011
DOI: 10.1002/dvdy.22595
free pdf: http://onlinelibrary.wiley.com/doi/10.1002/dvdy.22595/pdf

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. 
[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.]

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