Re: Ornithomimus had feathers and "display" winglike forelimbs

[Tim Williams <tijawi@gmail.com>]

> Darla K. Zelenitsky, François Therrien, Gregory M. Erickson,
> Christopher L. DeBuhr, Yoshitsugu Kobayashi, David A. Eberth, and
> Frank Hadfield (2012)
> Feathered Non-Avian Dinosaurs from North America Provide Insight into
> Wing Origins.


If all ornithomimosaurs had these big feathers on their forelimbs...
how long would the feathers on a _Deinocheirus_ be??


Anyway, Zelenitsky &c assign a non-aerodynamic function to the
evolution of pennibrachia, arguing that the wing-like structures of
_Ornithomimus_ would have been used for "reproductive activities (such
as courtship, display, and brooding) and were only later, among
maniraptorans, coopted for other roles, including flight."  Makes
sense.


In that light, it is possible that a function in brooding actually
fostered the development of vaned, bipinnate feathers that were
superficially aerodynamic.  John Ostrom in his _The Quarterly Review
of Biology_ essay (1974) argued that the same properties that made
pennaceous feathers suitable for flight also made them suitable for
insulation:

    "But it is important to remember that the vanes are also the
critical air-trapping surfaces of the insulating contour feather, as
well as the lifting surfaces of the remiges. Moreover, the
    rigid shaft provides the necessary leverage for the feather
muscles to fluff or compress the plumage. In other words, the basic
design of (modern) contour feathers is equally
    suited for over-lapping layers of adjustable, air-trapping
insulators and semi-rigid, light-weight, flight surfaces. The flight
feathers are merely greatly enlarged contour feathers-
    enlarged for the secondary purpose of flight. Those properties
that make them ideal insulating structures preadapted them as ideal
aerodynamic structures."


However, if large and pennaceous forelimb feathers were originally
used for brooding as "air-trapping insulators", then the shift to a
flight-related function would be simpler.  All that's needed is for
the vane to become asymmetrical.


The presence of long forelimb feathers might also explain the
appearance of the semilunate carpal wrist (which appears to have
evolved *after* ornithomimosaurs): It was simply to help fold the
forelimbs, and get the pennibrachia out of the way when they weren't
in active use.  No need for a flight-related explanation, or a
predation-related one.





Cheers

Tim