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How the pterosaur got its wings

Ben Creisler

A new online paper:

Masayoshi Tokita (2014)
How the pterosaur got its wings.
Biological Reviews (advance online publication)
DOI: 10.1111/brv.12150

Throughout the evolutionary history of life, only three vertebrate
lineages took to the air by acquiring a body plan suitable for powered
flight: birds, bats, and pterosaurs. Because pterosaurs were the
earliest vertebrate lineage capable of powered flight and included the
largest volant animal in the history of the earth, understanding how
they evolved their flight apparatus, the wing, is an important issue
in evolutionary biology. Herein, I speculate on the potential basis of
pterosaur wing evolution using recent advances in the developmental
biology of flying and non-flying vertebrates. The most significant
morphological features of pterosaur wings are: (i) a
disproportionately elongated fourth finger, and (ii) a wing membrane
called the brachiopatagium, which stretches from the posterior surface
of the arm and elongated fourth finger to the anterior surface of the
leg. At limb-forming stages of pterosaur embryos, the zone of
polarizing activity (ZPA) cells, from which the fourth finger
eventually differentiates, could up-regulate, restrict, and prolong
expression of 5′-located Homeobox D (Hoxd) genes (e.g. Hoxd11, Hoxd12,
and Hoxd13) around the ZPA through pterosaur-specific exploitation of
sonic hedgehog (SHH) signalling. 5′Hoxd genes could then influence
downstream bone morphogenetic protein (BMP) signalling to facilitate
chondrocyte proliferation in long bones. Potential expression of Fgf10
and Tbx3 in the primordium of the brachiopatagium formed posterior to
the forelimb bud might also facilitate elongation of the phalanges of
the fourth finger. To establish the flight-adapted musculoskeletal
morphology shared by all volant vertebrates, pterosaurs probably
underwent regulatory changes in the expression of genes controlling
forelimb and pectoral girdle musculoskeletal development (e.g. Tbx5),
as well as certain changes in the mode of cell–cell interactions
between muscular and connective tissues in the early phase of their
evolution. Developmental data now accumulating for extant vertebrate
taxa could be helpful in understanding the cellular and molecular
mechanisms of body-plan evolution in extinct vertebrates as well as
extant vertebrates with unique morphology whose embryonic materials
are hard to obtain.