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Tracing the Evolution of Avian Wing Digits

From: Ben Creisler

A recent paper not yet mentioned on the DML:

Xing Xu & Susan Mackem (2013)
Tracing the Evolution of Avian Wing Digits. [Review Article]
Current Biology 23(12): R538–R544

It is widely accepted that birds are a subgroup of dinosaurs, but
there is an apparent conflict: modern birds have been thought to
possess only the middle three fingers (digits II-III-IV) of an
idealized five-digit tetrapod hand based on embryological data, but
their Mesozoic tetanuran dinosaur ancestors are considered to have the
first three digits (I-II-III) based on fossil evidence. How could such
an evolutionary quirk arise? Various hypotheses have been proposed to
resolve this paradox. Adding to the confusion, some recent
developmental studies support a I-II-III designation for avian wing
digits whereas some recent paleontological data are consistent with a
II-III-IV identification of the Mesozoic tetanuran digits. A
comprehensive analysis of both paleontological and developmental data
suggests that the evolution of the avian wing digits may have been
driven by homeotic transformations of digit identity, which are more
likely to have occurred in a partial and piecemeal manner.
Additionally, recent genetic studies in mouse models showing plausible
mechanisms for central digit loss invite consideration of new
alternative possibilities (I-II-IV or I-III-IV) for the homologies of
avian wing digits. While much progress has been made, some advances
point to the complexity of the problem and a final resolution to this
ongoing debate demands additional work from both paleontological and
developmental perspectives, which will surely yield new insights on
mechanisms of evolutionary adaptation.