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Re: Deinonychus Morphological Variations within Ontogeny



Jason Brougham <jaseb@amnh.org> wrote:

> Agreed that non - avialan paravians did not fly like modern birds. Modern 
> birds can fly for 9 days and
> nights, over 11,000  kilometers, without stopping, eating, or resting even 
> once. (Limosa  lapponica
> baueri, Gill R. E, Jr, Tibbitts T. L, Douglas D. C, Handel C. M, Mulcahy D. 
> M, et al. Extreme endurance
> flights by landbirds crossing the Pacific Ocean: ecological corridor rather 
> than barrier? Proc R Soc B.
> 2009;276:447–457).


Yes, very impressive.  At the other extreme there are birds who have
extremely poor flight abilities, insofar as they can become airborne
but lack sustained flight: scrubbirds, lyrebirds, wattlebirds
(including the kokako), tapaculos, mesites, several rails, etc.  These
are birds that engage in short fluttering bouts of flight - such as to
escape a predator, or clear a hurdle on the ground, or reach a tree
branch.  There are also flightless birds that glide while on the
ground (kagu), or glide down to earth from trees (kakapo) - so no
powered flight, just "passive" gliding.  It may be that the incipient
stages of flight in theropods resembled one or more of these aerial
behaviors, observed in weakly-flighted or flightless-but-gliding birds
of today.


> But many of us are interested in how flight in the bird lineage began, and 
> rudimentary aerodynamic
> capabilities are of the greatest interest to us. A small theropod that could 
> leap and glide downhill away
> from danger is extremely interesting.


So do I (see above for possible modern analogs). This is the kind of
rudimentary aerial behavior that could have started flight evolution
in terrestrial theropods.  I certainly think it's a far more plausible
scenario than arboreal gliding (= gliding from tree to tree).


> (Especially considering that both quail and starlings produce 80 -
> 90% of their takeoff velocity from leaping, rather than flapping. Earls, 
> K.D.; (2000) The Journal of
> Experimental Biology 203, 725 - 739.) Personally I think that, looking at the 
> whole pool of basal
> paravians we have in Eosinopteryx, Xiaotingia, Anchiornis, Aurornis, and the 
> like, there is a great
> profusion of possibilities.


Take-off velocity was hindlimb-driven; the wings then continue the
motion initiated by the hindlimbs (including supporting the bird's
weight in the air), and sustain forward or ascending flight.

I particularly like Earls' thoughts on the origin of avian flight,
including her "jump-start" model.  As she says: "Any downward movement
of a feathered forelimb after the initiation of a leap could
potentially add height or distance to the ballistic path, regardless
of the reason for jumping."  The initial launch could be
hindlimb-driven, but the lift and thrust thereafter (required to
remain airborne) would both depend on the actions of the forelimbs and
pectoral musculature.  This could be a problem for basal (=
non-ornithothoracean) birds, irrespective of the range of excursion
the forelimbs were capable of.




Cheers

Tim