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Evolution and development of avian limb skeletal traits

From: Ben Creisler

A new paper that may be of interest:

Ryohei Seki, Namiko Kamiyama, Ayumi Tadokoro, Naoki Nomura, Takanobu
Tsuihiji Makoto Manabe and Koji Tamura (2012)
Evolutionary and Developmental Aspects of Avian-Specific Traits in
Limb Skeletal Pattern.
Zoological Science 29(10):631-644. 2012
doi: http://dx.doi.org/10.2108/zsj.29.631

The two sets of paired appendages, called limbs, are locomotory organs
in tetrapods that are used for various functions (e.g., walking,
running, crawling, digging, climbing, diving, swimming, and flying).
Unlike such organs as the eye, which contain specialized tissues such
as the lens and photoreceptor, the limb does not have any specialized
cells or tissues, but consists of common tissues, such as bone,
cartilage, muscle, blood vessels, and dermis. However, limb morphology
is highly specialized and varies to provide species-specific modes of
locomotion. As do the vertebrae and skull, the limb skeleton varies in
morphology among species. The diversity of limb skeletal morphology
provides examples of material for studies on morphogenesis. Avian
forelimbs have evolved into wings for flight. The skeletal pattern in
the avian limb has many traits that are unique among extant species of
vertebrates; some of such traits are avian-specific, others are shared
with more basal members of Theropoda, to which Aves belongs. Since
such avian traits generally form during ontogenic development,
determining when and how they appear in the developing embryonic limbs
or limb buds provides important insights into the mechanisms
underlying the generation of vertebrate morphological diversity. Here,
we present an overview of several features of the skeletal pattern in
the avian limb and discuss the developmental mechanisms responsible
for their unique and lineage-specific traits.