Re: Archaeopteryx not the first bird, is the earliest known (powered) flying dinosaur

[Tim Williams <twilliams_alpha@hotmail.com>]

Gregory S. Paul wrote:

> The basic contention that small theropods must have had a lot of 
> preadaptations on hand before becoming fliers is fundamentally absurd. 
> Something similar to Ornitholestes already has the basic skeletal features 
> needed to become an incipient glider.

It may be true that _Ornitholestes_ had sufficient hardware to become an 
incipient glider.  To me an "incipient glider" is a parachuter: the animal 
cannot yet produce an airfoil, but the skeletal proportions and integument 
are sufficient to slow and perhaps guide its descent to the ground.  But 
your hypothesis implies that the direct ancestors of flying theropods must 
have evolved flight in the trees.  I don't think we have yet reached the 
point where we can establish that birds did evolve flight with the 
assistance of gravity every step of the way ('trees-down', if you will).

You may argue that _Ornitholestes_ already had the basic skeletal features 
needed to become an incipient glider, but you need to explain (a) why 
_Ornitholestes_ would need to glide; and (b) how _Ornitholestes_ arrived at 
an elevation (e.g., a tree-branch) that allowed it to turn potential energy 
into kinetic energy - and commence gliding.  I know you are just using 
_Ornitholestes_ as an example, and that you are not implying that 
_Ornitholestes_ gave rise to birds; but my counter-arguments apply to any 
small theropod.  Every stage leading to flight has to be selectively 
advantageous.  If you contend that the ancestors of birds were arboreal 
gliders, then there must be a selective advantage to the theropod being 
arboreal, then becoming airborne, and then staying airborne.

> All it has to do is evolved sufficiently long wing, asymmetrical wing 
> feathers, hold the arms out to the sides and there you go.

But why would it need to do this?  What's missing is a *reason* to evolve 
all of these features.

> All the other stuff for folding the wings (elbow-wrist push-pulley system), 
> further increasing lift area (longer arms and still bigger wing feathers)  
> and producing a power stroke (large sternal plate, bigger pectoral crest, 
> ossified sternal ribs and uncinates) can be developed as flight progresses.

This may be true, but it requires that theropods passed through a gliding 
stage on the way to flight.  And you do not say why these features could 
*not* be pre-adaptations - like the elongated forelimb and manus, and 
ossified sternum, and short tail.  I am not saying that these characters 
*must* be preadaptations that evolved for a non-aerial purpose (like 
predation, or maneuverability on the ground) before being exapted for 
powered flight.  What I am trying to do is keep this question open - instead 
of asserting that these features *must* have been flight-related from the 
very beginning.  I think there is evidence that certain features (e.g., 
elongated forelimb and manus, ossified sternum, shorter tail, maybe even 
broad-vaned feathers) may have evolved prior to the advent of powered 
flight, and were later incorporated into the flight apparatus.  But I'm not 
being dogmatic on this point.

> No one has ever shown why any dinosaurian flight adaptation had to evolve 
> first as a preadaptation, aside from the combination of long arms and 
> bipedalism.

I don't want to get bogged down in the old 'ground-up' vs 'trees-down' 
dichotomy, but if the ancestors of birds evolved flight in a terrestrial 
setting then the requirements become a little steeper.  Unless the animal is 
fully arboreal, then the animal's own exertions are needed to get it into 
the air and keep it there.  The WAIR model, for example, requires an 
incipient flight stroke, which implies that the expanded wing elbow-wrist 
push-pulley system could have evolved prior to powered flight.  By contrast, 
a passive gliding stage is 'easier' in the sense that the evolution of a 
lift-and-thrust-generating stroke and more heavy-duty pectoral musculature 
can be deferred.  But I don't think we are at the point where we can dismiss 
the role of a terrestrial component in the evolution of avian flight, so all 
options are still on the table.

I personally favor a gliding phase as a prelude to powered flight, but this 
is just my intuition at work; and I cannot use my intuition alone to trump 
the work of people like Burgers and Chiappe and Dial who demonstrated (both 
theoretically and experimentally) that a 'ground-up' model of avian flight 
is feasible.

> One or more flight feature may have started as a preadaptation, but that 
> does not mean any or all
> had to.

I know - that's what I've been saying all along.  In a nutshell.

> Nor has anyone explained why Archaeopteryx has expanded muscle attachments 
> areas on its arms just to glide.

I wasn't arguing that _Archaeopteryx_ was a passive glider.  However, I was 
arguing that the expansion of muscle attachments *might* be a preadaptation 
to flight, and were inherited by birds from theropod forbears that used 
their powerful arms for catching and holding prey.

> It is obvious that expansion of muscle attachments on a forewing will one 
> way or another allow and improve the power of a flight stroke.

By the same token, it is obvious that these same adaptations might allow a 
predator to better hold onto large prey - like a _Velociraptor_ trying to 
grapple with a _Protoceratops_, for example.  Or a _Deinonychus_ holding 
onto a bucking _Tenontosaurus_.

> I truly do not even begin to understand the arguments to the contrary. They 
> seem part of a continuing effort to some, based on a historical heritage 
> that saw dinosaurs as having nothing to do with birds, to keep theropods 
> that were not full birds as nonfliers or mere gliders, as though nonavian 
> dinosaurs were for some reason not allowed to be true fliers. Very odd.

Speaking for myself, as someone who has absolutely no doubt that birds 
evolved from theropods (and so, therefore, *are* theropods), I can say that 
I completely disagree with this statement.  And there is nothing 'mere' 
about gliding.  Successful gliding is an aerodynamic feat that requires a 
considerable anatomical investment on the part of the glider.

Cheers

Tim