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Re: Avian Ancestors, new book on theropods



Ruben Safir <ruben@mrbrklyn.com> wrote:

> Right, after using powered flight.  I think those feathers and wings had
> serious lift.  You line those feathers up, your going to generate serious
> lift in the slightest breeze.


I think we might be talking across each other to some extent.  To
achieve powered flight you need thrust, not just lift.  I'm pretty
sure that the wings of _Archaeopteryx_ were capable of lift.  Same for
the wings of _Microraptor_, _Jeholornis_, etc.  Maybe even
_Anchiornis_ too (which has short, blunt, symmetrical remiges).  But
to make the transition to powered flight, the wings need to generate
thrust as well as lift.  Otherwise flight is not sustained (or
"powered").


> I'd need to be proved that one needs a complete flap to create lift with
> a full set of flight feathers.


But one doesn't need a flap to achieve lift.  The flap generates thrust.


In today's world, there are a lot of mammalian gliders that generate
lift.  These particular gliders show no inclination toward powered
flight.  Presumably at least one group of long-extinct mammalian
gliders made the transition to powered flight; these went on to become
bats (Chiroptera).  But lift and powered flight are not the same
thing; the former is a component of the latter.


> It doesn't make sense to me.  I can so no reason
> why these things
> couldn't get lift even with a small gust of wind.


They might well have done.  But there is more to powered flight than lift.


> It really only makes sense if powered flight was used for, and then the
> feathers developed
> in an asymmetrical manner.  What possible evolutionary force would
> create the reverse?


One hypothesis is that the asymmetrical vane evolved in response to
the use of the feather in lift generation.  Once lift was achieved,
the wing could subsequently be employed to generate thrust.  If this
is the trajectory by which powered flight was achieved (and it is just
one of several competing hypotheses), then there is no reason to
assume that powered flight came *before* the evolution of the
asymmetrical feather.  It's also not clear where in this trajectory
_Archaeopteryx_ belongs - if anywhere at all.


Speaking of the flight stroke...


David Marjanovic <david.marjanovic@gmx.at> wrote:


> Are you sure about that last point? As preserved (and that only in the Berlin 
> specimen), the sternum was
> very small; but can we tell if the whole thing calcified?
>
> And on the other side of the pectoral muscle, the deltopectoral crest is 
> huge. This also allows large
> deltoid muscles for raising the wings -- bats use their deltoids for the 
> upstroke.


So did pterosaurs.  The same might be true for basal avialans (birds).
 If _Archaeopteryx_ could raise its humerus above the back, and
execute a full flight stroke, then the deltoideus (rather than the
supracoracoideus) would be the principal wing elevator.  A
deltoideus-driven upstroke may therefore be primitive for birds.


But _Ichthyornis_ also has a fairly prominent deltopectoral crest
*and* a huge keeled sternum, with the latter indicating advanced
flight capabilities.  I'm reluctant to put to much stock in the large
deltopectoral crest as an indicator of powered flight in basal
avialans, for two reasons: (1) it's something that they inherited from
non-flying theropods, and (2) the supracoracoideus is the major wing
elevator in crown birds.


>> AFAIK, there is no widely-held belief that the asymmetrical feather
>> evolved only once.
>
> It's at present the most parsimonious option, though, isn't it?


It depends on the topology of the phylogeny, such as the position of
_Archaeopteryx_ relative to the line that led to crown birds.  There
are studies (e.g., Longrich et al., 2012) that have raised alternative
options.





Cheers

Tim