PTEROSAURS AS AVIAN RELATIVES

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

Larry Febo wrote:

<<How would you account for the "pneumatic foramina", the hollow bones 
that are virtually identical in both birds and pterosaurs? And the 
braincase being "avian" in character? I suppose you think that these 
also were the result of convergence?>>

As Terry Jones pointed out to me long ago, pterosaurs, unlike birds, 
seemed to not have pneumatopores in the limb bones but only in the 
elongate finger bones.  This is rather unlike the avian system of 
pneumatization and suggests that pterosaurs did not hollow out bones 
through their respiratory system, but rather some other way.  Anyway, 
even if true, it can be considered a flight adaptation because it can 
obviously lighten weight to a point.  Interestingly, early birds seem 
not to excavated their limb bones with pneumatopores, this development 
seems to be a strict ornithurine or even neornithine character.  

Pterosaur braincases have never to my knowledge been considered "avian".  
The only obvious avian character is the inflation of the braincase, but 
this is not as well developed as in theropods.  Pterosaurs seem to lack 
the characteristic three tympanic recesses of birds and theropods and 
convergently crocodylomorphs.  This is not to say that the recesses were 
a later development, but it seems likely that these are primitive avian 
characteristics since they are present in _Archaeopteryx_.  

Yes, I do think that these characters do not link pterosaurs and birds 
any more than they can link any other two disparate groups.  These two 
characters are poorly described and do not show a distribution that 
makes them likely as synapomorphies of the pterosaur-bird clade. Hell, 
even Haematothermia has more osteological characters of greater weight 
than these.  

<<That birds hit on the same idea,... to hollow out the bones and make 
them part of the respiratory system? ( to me...also a somewhat complex 
development). The more points you claim are convergent, the less likely 
I am to believe it.>>

Convergence happens.  As Darren Naish (actually Michael Lee) pointed out 
a few days ago, snakes and amphisbaenians and dibamids converged on each 
other because limblessness and burrowing habits.  Amphisbaenians could 
be the sister group to snakes, they do show some scolecophidian 
characters, but this relationship is far outweighed by the 40 odd 
characters linking snakes and mosasaurs in Pythonomorpha.  To avoid this 
convergence, all we know about scincomorphs, thecoglossids, platynoans, 
and varanoids we have to be turned around.  Convergence happens.

<<I believe the common ancestor was small, insectivorous (small 
cranium), had furcula as well as acrocoracoid process (enabaling rapid 
wingbeat allowing for controlled perched landing), typical pterosaurian 
flight membrane along with (of course) integumentary fibres.>>

Early birds (_Archaeopteryx_) and avian relatives (dromaeosaurs, 
_Protarchaeopteryx_, _Caudipteryx_) lack a true acrocoracoid process 
(they had the precursor, the biceps tubercle of Walker).  

In fact, early birds do not have the elongate coracoid in the manner of 
pterosaurs.  Your character of the elongate coracoid shared between the 
two groups is more likely a convergent adaptation since basal birds lack 
this character (unless they lost it convergently).  The elongation of 
the coracoid, as shown ably by Tarsitano in the Archae. Conf. Volume 
(1985) is a character related to the deepening of the thorax for various 
reasons related to flight.  The supracoracoideus wing abductor system 
(which is rather different in the two groups) is related to the 
elongation of the coracoid according to Tarsitano.

Matt Troutman 
m_troutman@hotmail.com

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