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Origin of bird flight: ontogenetic-transitional wing (OTW) hypothesis
Kenneth P. Dial, Brandon E. Jackson & Paolo Segre (2008). A fundamental avian
wing-stroke provides a new perspective on the evolution of flight.
Nature advance online publication 23 January 2008 | doi:10.1038/nature06517;
Received 20 August 2007; Accepted 27 November 2007; Published online 23 January
"The evolution of avian flight remains one of biology's major controversies,
with a long history of functional interpretations of fossil forms given as
evidence for either an arboreal or cursorial origin of flight. Despite
repeated emphasis on the 'wing-stroke' as a necessary avenue of investigation
for addressing the evolution of flight, no empirical data exist on wing-stroke
dynamics in an experimental evolutionary context. Here we present the first
comparison of wing-stroke kinematics of the primary locomotor modes (descending
flight and incline flap-running) that lead to level-flapping flight in juvenile
ground birds throughout development (Fig. 1). We offer results that are
contrary both to popular perception and inferences from other studies.
Starting shortly after hatching and continuing through adulthood, ground birds
use a wing-stroke confined to a narrow range of less than 20°, when referenced
to gravity, that directs aerodynamic forces about 40° above horizontal,
permitting a 180° range in the direction of travel. Based on our results, we
put forth an ontogenetic-transitional wing hypothesis that posits that the
incremental adaptive stages leading to the evolution of avian flight correspond
behaviourally and morphologically to transitional stages observed in
Later in the paper appears this brilliant statement, which hits the nail right
on the head...
"Perhaps we can cut the Gordian knot created by the false dichotomy of the
highly charged, but unresolved, cursorial–arboreal debate. The OTW hypothesis
embraces salient features of both the arboreal and cursorial hypotheses yet
clearly differs from both."
Although I don't necessarily agree with everything in the paper (only ~
90-95%). For example, I'm a bit skeptical about this...
"Commonly held assumptions within the cursorial school about the plausible
function of proto-wings are inconsistent with the ontogenetic biology of extant
forms; for example, no extant species uses its wings to run faster, to secure
prey or run–glide."
No extant avian uses its forelimbs to catch prey, because these forms already
have the forelimbs highly modified into wings; or else they are flightless
birds that evolved from forms that had the forelimbs highly modified into wings.
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