RE: PTEROSAURS: AVIAN ANCESTORS?

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

<<Yes, I can see in the anatomy of bats a good example of convergence. 
Pterosaurs and birds however appear, to me,  to be more than just 
parallel evolution.>>

Why?  

Pterosaur shoulder girdles are not similiar to bird shoulder girdles in 
many details.  For one, the pterosaur scapula is shorted and is fused to 
the coracoid.  The sterni of birds and pterosaurs are very different 
too.  Pterosaurs have a seperate element (the cristospine) immediately 
anterior to the sternum that functions like the carina of the bird 
sternum.  However, it is very different in placement, shape, and nearly 
everything else.  Pterosaurs also have sternal ribs that are more 
numerous than bird sternal ribs and lack the joints of bird sternal 
ribs.  

<<My view that these maniraptoriforms are descended from arboreal avian 
forms would account for a similar, non-convergent arrangement in these 
pectoral girdles as well.>>

No fossil evidence really backs this up.  Plus, it is very clear that 
maniraptoriforms are descended from coelurosaurs, which are descended 
from avetheropods, avetheropods are descended from tetanurines, 
tetaurines are descended from theropods, theropods are descended from 
basal dinosaurs, basal dinosaurs are descended from dinosauromorphs, 
dinosauromorphs are descended from ornithodirans (if pterosaurs are 
outside Archosauria than this definition would be different), 
ornithodirans are descended from a basal archosaur, that basal archosaur 
was descended from some archosauromorph, and so on.  Even if one of 
thses steps is false (keep in mind I simplified it a bit) it would still 
hold up barring a future analysis that suggests otherwise. 

<<Wellnhofer states that this overlapping arrangement of the coracoids 
is typical of the Ramphphorynchcoidea, but that the Pterodactyloidea 
sport a symmetrical juxtaposition on the sternum. (It`s interesting that 
the pterodactylodia are also the more "birdlike" in many other features 
as well).>>

Rhamphorynchoidea is paraphyletic; an assemblage of basal pterosaurs.  
Its a long way from the pterodactyloid structure to birds.  

>So are many pterosaur bones, including the skulls.

Hollow bones are found in most all theropods (including nearly all skull 
bones).  Primitive. 

>All of them?  Some of them??  Isn`t this still a debated issue?

If prolacertiforms are the outgroup, it is primitive for Pterosauria.

<<P.S. don`t forget, most of the pterosaur fossils we have are already 
quite advanced and specialized, too much  so to be exactly analogous to 
birds in all features.>>

Exactly.  They are too specialized to be avian ancestors.  We have much 
better evidence from the theropods.

>No indication of even a fourth metacarpal? Then what about >Protoavis??

Of an elongate (pterosaur-elongate) fourth digit.  

Matt Troutman

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