Re: bottom decker?? pteros

[david peters <davidrpeters@earthlink.net>]

> 
> Just wondering, and I'll discuss this further if there's any interest
> out there, but could it be that these bottom decker scapulocoracoids are
> simply inverted?

I don't know.  I've seen some reconstructions in the past that were
inverted.

I've just pulled out a Qsp sc and without measuring it, just by
uncalibrated eyeball I'd say that the socket is roughly about 55% of the
way down from the top of the sc.  If you add the sternum to the vertical
height, maybe 45% of the way down from the top?

There's been some discussion that these 'lowwingers' have decreased
stability.  But I have 3 highwing airplanes and one lowwing.  The
lowwing is by far the most stable.  There are other factors that are far
more important to stability than the vertical location of the cg
relative to the shoulder articulation.

Jim


>>>>>
Well, Jim, the article actually considers Q a middle-decker and a Tapejara-like 
scap/cora to be a bottom-decker. In this latter configuration the "coracoids" 
extend laterally from the sternum, much like scapulae do on top-deckers. That's 
why I wondered.

Now the key ingredient in determining scapula from coracoid in these cases 
appears to be the double-headed or forked articulation of the "coracoid" which 
matches the same configuration of a genuine coracoid in Quetzalcoatlus. And 
this is, no doubt, one of the purported "synapomorphies" uniting the 
Azhdarchidae to the Tapejaridae [haven't seen the papers yet].

Looking around for good counter samples has problems. Sinopterus offers a great 
pair of articulated scap/coras but it is crushed dorsoventrally so arguments 
could fly both ways. Unfortunately no scapulocoracoids are known [to me at 
least] from the real outgroup [in my cladogram at least] to the Tapejaridae: 
the Dsungaripteridae. The best we can do is look at the Germanodactylids and 
the Pteranodontids, including Nyctosaurus, and here some clues in 
characteristic shapes can be found. G. cristatus (No. 61 of Wellnhofer 1970) 
has a morphologically similar scapulocoracoid to the questionable Tapejarid, 
but it is disarticulated. As we go phylogenetically beyond these taxa the 
morphological similarities start to fade. 

Two cladistic facts do seem to emerge however. First: the clade that includes 
azhdarchids and their ancestors among the dorygnathids appears to have a 
coracoid that flares ventrally, while the clade that includes all of the 
sharp-nosed germanodacs, tapejarids, pteranodontids, etc., does not have this 
flare.  Second: the pterosaurs which appear to be adapted to soaring, such as 
the ornithocheirids in one clade and the pteranodontids and nyctosaurs in 
another, appear to have a shorter scapula than coracoid, and thus would all be 
top-deckers.

Airplanes with high or low wings usually have a reason other than flying for 
them. Sea planes often have high wings to keep water away. Fighters have low 
wings so pilots can see the skies and the planes can have short landing gear.  
For private pilots, it seems the main difference comes in visibility (seeing 
the ground beneath the wings or not) and in ease of maintenance (preflighting 
under the wing in a stance or a deep crouch). Dihedral is important, but it 
varies every nanosecond in a flapping pterosaur. Most pterosaurs appear to have 
been top-deckers, like birds are. The reason Quetz. has a lower glenoid has 
more to do with its terrestrial adaptations. In my opinion, which differs from 
Jim's, Q. spent alot of time in water where it was a bottom-feeder aided by its 
huge size, long rostrum and long neck, rather than feeding while soaring. And 
it came from a long line of similarly adapted pterosaurs of phylogenetically 
increasing size, driven by competition into untapped ni!
 ch!
es in ever deeper waters.

David Peters
St. Louis