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

[Tim Williams <twilliams_alpha@hotmail.com>]

Gregory S. Paul wrote:

> Tim W offered a hypothesis for explaining how the well developed arms and 
> wings of Jehol dromaeosaurs may have evolved for enhanced predation rather 
> than flight more advanced than Archaeopteryx. This concept is a nonstarter.

I beg to differ.  :-)

> I forgot to mention that a character present in the dromaeosaurs is 
> stiffening of the central digit associated with the flattening of the same 
> finger (the central finger of Archaeopteryx remained more flexible, 
> although probably less than the theropod norm). The reduction or loss of 
> flexation of the main finger is of course adaptative if the arm is being 
> enhanced as wing, but not at all for
> predation.

If you replace the word "predation" for "manipulation" then you would be 
right.  However, the fact that dromaeosaur fingers were longer and more 
inflexible than those of most other non-avian theropods should not 
necessarily be seen as maladaptive for predation.  First of all, 
_Deinonychus_ and _Velociraptor_ have the same inflexible main finger as 
_Microraptor_ and _Sinornithosaurus_, and it is clear that _Deinonychus_ and 
_Velociraptor_ were predators.  Alan Gishlick has done some good 
biomechanical studies on the arms of _Deinonychus_ and demonstrated that the 
hands would have been quite effective at grasping prey in a two-handed 
fashion.   Thus, the loss of the hand's ability to *manipulate* prey does 
not mean that the hand could not be used for predation.  Now, perhaps the 
inflexibility of the main finger did originally evolve as a flight-related 
feature (this is quite possible); but its presence/retention in 
ground-dwelling predators like _Deinonychus_ argues against this feature 
being limited to aerial dromaeosaurs, and thus argues against this feature 
being directly flight-related.  (Stiff and inflexible fingers are also seen 
in at least some oviraptorosaurs.)

Secondly, Greg Paul argues that almost all flight-related features seen in 
birds (including _Archaeopteryx_) originally evolved for the purpose of 
powered flight.  I have a conceptual problem with this, especially when it 
comes to reconstructing pre-flight behaviors.  Under Greg's view virtually 
every character of advanced maniraptorans (longer arms, longer hands, large 
ossified sternum, short tail, broad-vaned feathers, etc) can be tied to 
flight.  However, if all these characters evolved specifically *for* flight 
and not before, that leaves us with few (if any) characters that could be 
acted on by natural selection during the pre-flight stages.  In other words, 
what were theropods doing before they could fly that allowed this transition 
from non-flight to flight to occur in the first place?  Greg's hypothesis 
says a lot about what characters theropods used to fly, but nothing about 
what characters were recruited during the incipient stages of flight.

To me, this is the advantage of predation-to-flight models: they provide a 
starting point for the transition to flight by providing the raw material 
for evolution (by exaptation).  The 'predatory stroke' hypothesis has its 
flaws, as does the Ostrom's "insect-net" hypothesis that came before it.  
But at least they both provide a model by which incipient flight behavior 
evolved, and suggest that some flight-related features originally evolved 
for another purpose: catching prey.

> The basic idea that such extremely powerful arms were for predation is 
> contradicted by the fact that as soon as larger dromaeosaurs lost flight 
> they reduced the arms to more normal dimensions.

This is a circular argument, given that you begin with the premise that 
larger dromaeosaurs were in fact secondarily flightless.

> The wings of sinornithosaurs were certainly inferior to those of flightless 
> dromaeosaurs as hunting organs.

I don't see how, unless you argue that the long forelimb feathers interfered 
with hunting - which is possible, but (of course) difficult to test.

> The hypothesis that sinornithosaur arms evolved their flight features for 
> enhanced predation is so inferior that it is not viable. It is also 
> irrelevant. Even if sinornithosaur arms really were adapted for predation, 
> this would still leave the very powerful, fully winged arms superior as 
> flying organs than those of Archaeopteryx! And inferior in terms of finger 
> flexibility.

You may be right to claim that sinornithosaurs were better fliers than 
_Archaeopteryx_ - although I'm certainly not conceding this point.  I prefer 
the hypothesis that sinornithosaurs were passive gliders, not powered 
fliers.  However, I do not think it is wise to view theropod relationships 
solely through the prism of flight evolution.  It is possible that 
sinornithosaurs show more (and/or superior) flight-related features compared 
to _Archaeopteryx_, and that sinornithosaurs are more basal than 
_Archaeopteryx_ relative to modern birds; the two are not mutually 
exclusive.  As I said above, not all flight-related characters may have 
begun as flight-related characters.  And besides, phylogenetic analyses take 
into account more than just those characters that are perceived to be 
flight-related.

> As I explained in DA the flight adaptations found in the bird-like 
> theropods are generally difficult to explain in a nonflight context,

No, they are not difficult at all to explain in a nonflight context.  I 
cannot see why it is so astonishing that a group of bipeds that use their 
arms to catch prey would evolve longer and stronger arms for the purposes of 
improving (or refining) their prey-catching ability.

> Small, long armed theropods were already pre-adapted to becoming fliers,

Maybe I'm being limited by my own imagination, but I do not see how being 
"small" and "long-armed" is enough for natural selection to act upon in 
order to convert a non-flier into a powered flier.  The way I see it, the 
process was far more complicated at that.  I see a far bigger role for 
exaptation in the evolution of avian flight, and I cannot accept that *all* 
flight-related features only appeared when the animal was already a powered 
flier, or very close.

Cheers

Tim