PTEROSAURS: AVIAN ANCESTORS?

["Matthew Troutman" <m_troutman@hotmail.com>]

<<Other "alternate" theories have their proto-bird models. George 
Olshevsky points towards a thecodont closely related to Cosesaurus, 
Megalancosaurus and Longisquama.>>

I think that George thinks that these animals are basal theropods, or 
near basal theropods.  I also think that he only fews these as evidence 
of a arboreal 'dino/bird' clade.

 Larry Martin views these animals as crocodylomorphs based on several 
features.  However, the majority of people think that _Cosesaurus_ and 
_Megalancosaurus_ are prolacertiforms.  Dave Peters views _Cosesaurus_ 
as the sister-group to pterosaurs.  

<<Tarsitano and Hect also point to such a thecodont ancestry.>>

Which is exactly the same as the one that you described above.  

You forget Martin and Whetstone's (again, what happened to Kenneth 
Whetstone after 1983?  Did he bow out of paleo because of finances?  
Loose his appetite for research?  What?) crocodylomorph hypothesis.  
Basically, crocodilians and their nearest relatives share several 
features with birds that theropods lack, or were found in few theropods.  
However, most fossils of theropods suggest otherwise.  

<<I, on the other hand, prefer to think it was a very early, primative 
pterosaur that gave rise to a sister group that would eventually become 
aves.>>

Why?  The spark for pterosaur aerial ability was their elongate manual 
digit IV (for V) and its membrane.  Something that even early birds 
lack.  

<<To me, they share too many similar features to have been the cause of 
"convergence". (I.e. similar pectoral girdles with supracoracoideus 
function,hollow bones,  similar brain structure). Plus, they occur at 
just the right time to be ancestral to birds. Plus, if they did exist 
just prior to the fully feathered avian form, wouldn`t their very 
presence be a competitive obstacle to formation of a new avian type 
"from scratch". (This also is in opposition to a "ground-up" model of 
flight evolution).>>

Problems:

1)  The pectoral girdle of pterosaurs was not similiar to the pectoral 
girdle of birds.  Yes, it is similiar to the flight apparatus of birds 
with a slender scapula, elongate, strut-like coracoid, and an outwardly 
facing glenoid.  The problem with this corellation is that all of the 
features stated above are NECESSARY features for flight.  Consider bats 
(I'll refer to them collectively even though I think that Chiroptera is 
diphyletic)...  Bats have a clavicle that functions much like an avian 
coracoid; it stands nearly vertical, it is strut-like, and it 
articulates similiarly with the sternal elements.  The acute angle 
between the scapula and coracoid in birds and pterosaurs is also 
approached in bats, just substitute clavicle for coracoid.  

Further complicating the issue is the fact that most maniraptoriforms 
have a similiar arrangement in the pectoral girdle as birds.  

Early pterosaurs also have a unique coracoidal articulation to the 
sternum; the coracoids are articluated one behind the other.

2)  Theropods have hollow bones.  

3)  Pterosaur brains are not very similiar to bird brains (no pun 
intended).  They are rather similiar to croc brains in many details.  
Theropod brains are more similiar to bird brains.  

4)  There is really no "time" problem in the theropodian=>bird 
hypothesis.  Early maniraptorans are springing up all over.  

5)  Not all advocates of the theropod=>bird hypothesis believe that 
flight evolved from the ground-up. 

6)  There is lots of evidence (as outlined by Dave Peters) that suggests 
that pterosaurs are not related to dinosaurs, or are even in the 
Archaosauria.  Peters (following Wild) suggest that pterosaurs are 
descended from prolacertiforms, a group that might be close to either 
Archosauria or Lepidosauria.  

<<As for the missing furcula, or even clavicles in pterosaurs,  I 
attribute this to most pterosaurs being "top-heavy" in form, i.e. large 
crania relative to body size. Such top-heaviness would have made for 
fairly hard landings upon their forelegs, involving stresses that would 
have broken any clavicles (if in existance).>>

This could well be true, but prolacertiforms seem to lack clavicles.

<<Therefore, evolution would have eliminated them in the true pterosaur 
form. But if there existed a smaller, more primative form, with much 
smaller cranium (insectivior), more adapted to making perched landings 
on its hind legs, it would have been able to retain the furcula to aid 
its powered flight. Such a smaller form would be able to land on, and 
negotiate the more intricate branches of the gymnosperms, and newly 
evolving angiosperms, where larger pterosaurs couldn`t follow. These 
newer trees could invade colder enviorns, where insects were sure to 
follow, and in turn, so would these smaller, insectivorous pterosaurs. 
In order to adapt, these proto-birds had to develope feathers, at first 
just for insulation. Then, hairy proto-feathers evolving into contour 
feathers (to keep the body streamlined), finally into true flight 
feathers, to aid both in lift and maneuverability. There would have been 
definite selectionary pressure against a long wing digit in this colder 
enviorn,(Bergman`s Rule), and hence its eventual loss. It could have 
been a gradual exchange, a trade-off, loss of length of wing digit for 
increased length of flight feathers until finally...a bird!>>

1)  The pterosaur form was just fine the way it was.  

2)  This scenario assumes that bird ancestors had a large amount of 
aerodynamic control because hind leg first landings are hard to 
accomplish.  

3)  It seems to speculate that pterosaurs did not have insulation or 
pelage, which they did.  Now, feathers are not better insulators as hair 
so there is no reason to select for them, even for aerodynamics since 
gliding membranes are still present.  

4)  There is no evidence that early birds were endothermic.  

5)  Pterosaurs were already stream-lined with their "hair".  

6)  Pterosaur hindlegs seen to have been bound in a uropatagium, 
something that birds lack.  

7)  Pterosaur legs seem to have been bowed to a certain extent.  Some 
workers think that early bird legs were bowed to an extent, but not to 
the extent seen in pterosaurs.  

8)  The pterosaur acetabulum is not fully perforate.  

9)  If pterosaurs did go through a process such as this, there is little 
reason why the elongated digit IV should be lost.

10)  There is no evidence embryologically that birds had an arrangement 
in their manus like the pterosaur wing.  

<<Anyway, in looking for possible negative evidence that might test this 
hypothesis, I was once told by my old professor of evolution, (Max 
Hect), that..."it`s all in the wrist!" Even Dr. Kevin Padian, who was 
kind enough to review my hypothesis, told me..."it`s a long way from a 
pterosaur wrist to a bird wrist." I`m not sure exactly what this means, 
(I`ve seen evidence of the vertebrate structure doing some pretty 
spectacular "morphing").>>

Pterosaur wrists seem to been rather specilized.  The elements do not 
seem to be analogs to the cuneiform, scapholunar, and semilunates of 
bird wrists.  _Archaeopteryx_ has four carpal elements.  Pterosaurs have 
two or three.  

So, in conclusion, it seems that pterosaurs and birds share no real 
similiarities other than classic flight features. 

Matt Troutman

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