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Avian flight stroke origin

From: Ben Creisler

New in PLoS ONE (free pdf, of course):

Dececchi, T.A. & Larsson, H.C.E. (2011) 
Assessing Arboreal Adaptations of Bird Antecedents: Testing the Ecological
Setting of the Origin of the Avian Flight Stroke. 
PLoS ONE 6(8): e22292. 

The origin of avian flight is a classic macroevolutionary transition with
research spanning over a century. Two competing models explaining this
locomotory transition have been discussed for decades: ground up versus
trees down. Although it is impossible to directly test either of these
theories, it is possible to test one of the requirements for the trees-down
model, that of an arboreal paravian. We test for arboreality in non-avian
theropods and early birds with comparisons to extant avian, mammalian, and
reptilian scansors and climbers using a comprehensive set of morphological
characters. Non-avian theropods, including the small, feathered
deinonychosaurs, and Archaeopteryx, consistently and significantly cluster
with fully terrestrial extant mammals and ground-based birds, such as
ratites. Basal birds, more advanced than Archaeopteryx, cluster with extant
perching ground-foraging birds. Evolutionary trends immediately prior to
the origin of birds indicate skeletal adaptations opposite that expected
for arboreal climbers. Results reject an arboreal capacity for the avian
stem lineage, thus lending no support for the trees-down model. Support for
a fully terrestrial ecology and origin of the avian flight stroke has broad
implications for the origin of powered flight for this clade. A terrestrial
origin for the avian flight stroke challenges the need for an intermediate
gliding phase, presents the best resolved series of the evolution of
vertebrate powered flight, and may differ fundamentally from the origin of
bat and pterosaur flight, whose antecedents have been postulated to have
been arboreal and gliding.

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