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Re: Deinonychus Morphological Variations within Ontogeny



Jason Brougham <jaseb@amnh.org> wrote:


> What scenario do you prefer, then, logically, for the origin of flapping 
> flight? At one point
> or another you have discounted all of the transitional states that have been 
> proposed, saying
> all of them can only happen after the acquisition of the full powered flight 
> apparatus: not
> arboreal perches (gravitational acceleration), not WAIR (impossible for the 
> humerus to be
> elevated).


I'm not opposed to gravity-driven aerial locomotion from an arboreal
perch.  Not at all.  I just don't think that the animal in question
was arboreal.  (By "arboreal" I mean spending most or all of its time
in trees, which is the definition of arboreal.  For example,
clambering up trees and returning to earth is not arboreal - it's
scansorial.)  I've said as much here:

http://dml.cmnh.org/2015Mar/msg00100.html

I have reservations about WAIR, due to the requirement for a fairly
'modern' musculoskeletal component of the flight apparatus and
high-amplitude wingbeat (even accounting for more emphasis on
anteroposterior excursion).  But I really like the ability of WAIR to
negotiate steep surfaces, so it works for a small terrestrial
theropod.  So if the anatomy of non-avialan theropods turns out to be
compatible with WAIR, then great.


> Also not leaping takeoff and flapping, and not leap - gliding (just tonight,
> restricting these to ornithothoraces without any justification).


I stipulated Ornithothoraces because at this level we have good
evidence (osteological, inferred musculature, feathers/integument) of
a fairly derived lift- and thrust-generating flight apparatus.  For
more stem-ward lineages, the evidence is ambiguous.  Basal birds could
certainly have engaged in weak flapping flight - perhaps similar to
modern birds that have poor flight abilities (and in some cases may be
in the process of losing flight).  Hoatzins have atrocious flight
abilities, but manage to commute from tree to tree with their clumsy,
flapping flight.  So why not sapeornithids?


> Perhaps you suggest that Iberomesornis and Archaeorhynchus (the most basal 
> ornithothoraceans)
> were the the first to experiment with any aerodynamic behaviors?


Gosh, no!  I wouldn't suggest this at all.  I suggest that basal birds
and related pennaraptoran theropods (_Microraptor_ etc) experimented
with aerial behaviors - I've said as much in the past.  But as for
sustained *powered* flight - this is much more challenging, and this
particular behavior might not have evolved prior to Ornithothoraces.
Critters like _Protopteryx_, _Iberomesornis_ and _Archaeorhynchus_
were probably bona fide powered/flapping fliers.  I'm less sure about
_Archaeopteryx_, _Confuciusornis_, _Jeholornis_ and _Sapeornis_.


> Such as parachuting? Then we are faced with a  scenario where animals had 
> developed extremely
> long wings with asymmetrical feathers, tiny body size, fully reversed 
> halluces, and huge
> sternum muscles, all BEFORE they made any attempts to flap.


Parachuting or gliding might have been behaviors used by non-avialan
pennaraptorans.  Basal ornithuromorphs and enantiornitheans almost
certainly graduated well beyond this point, into sustained/powered
fliers.  As you say, they also had fully retroverted halluces (though
possibly achieved in different ways - the morphologies of the first
metatarsal are quite different between perching enantiornitheans and
crown birds).


> I must repeat that pterosaurs and bats have never had any problem flapping 
> without a triosseal
> canal or sternal keel.


I thought bats and pterosaurs have/had a keel?  I'm not going to go
into the entire supracoracoideus versus detoideus thing, but I've got
nothing against incipient flight in bird ancestors having a greater
role for a deltoideus-driven upstroke, and the development of the
triosseal canal being affiliated with increased pre-eminence the
supracoracoideus:

http://dml.cmnh.org/2013Mar/msg00013.html

http://dml.cmnh.org/2012Sep/msg00030.html


> In some cases you seem to venture up to the brink of logic (as in noting that 
> ANY downward
> motion of the wing would add lift), but then you retreat to a strange, ex 
> post facto position
> (arguing that only things that have advanced flying capabilities can be 
> models for the
> incipient stages of flight). In fact, reading back, it appears that you must 
> dispute that
> paravians are the ancestors of the birds, is that correct?


Nope, I never said any of that.  I've said (and repeated above) that
certain extant birds that have poor flight abilities might serve as
analogs for the incipient stages of avian flight.  I'm not certain
these qualify as "things that have advanced flying capabilities" - it
very much depends on what you mean by "advanced".

It also depends on what stage of flight evolution you're referring to.
The first aerial behaviors might have been entirely terrestrial or
scansorial.  In a later stage, whereby the flight stroke was refined
for sustained flapping flight, the theropod might have been fully
arboreal.


> Because you do not seem to see
> wings, asymmetrical feathers, hugely long arms, or tiny body size, as even 
> remotely indicative
> of aerodynamic behaviors.


Aerodynamic behaviors - yes.  Powered flight - not necessarily.

BTW, on the "asymmetrical feathers" issue, check out Feo et al.
(http://dx.doi.org/10.1098/rspb.2014.2864)


> Please state your hypothesis clearly and succinctly so that we on DML may 
> critique it.


Here's a start:

http://dml.cmnh.org/2015Mar/msg00083.html

http://dml.cmnh.org/2015Mar/msg00093.html

http://dml.cmnh.org/2015Mar/msg00100.html

http://dml.cmnh.org/2015Feb/msg00087.html




Cheers
Tim