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PTEROSAURS, PROTOBIRDS, AND CONVERGENCE



REGARDING:  Differences between bird and pterosaur shoulders

<<I`m not sure if these details aren`t related to the more powerful 
musculature needed by these pterosaurs to move their enormous wings 
without collapsing the ribcage.>>

All flying vertebrates have to have some sort of bracing to resist the 
compressive forces generated by the flight muscles.  All go through 
different ways.  How they do it probably depends on what kind of animal 
they are and their ancestor (Hill and Smith 1984; Flight Chapter).  

<<These might be  the features expected in a more advanced, and hence 
specialized form, not the more primitive, generalized form that birds 
might have evolved directly from. Even Wellnhofer states how even the 
earliest pterosaurs recorded seem already specialized to their 
particular mode of flight , and a great gap existing in the fossil 
record as to what exactly led up to the first pterosaurs,(as we know 
them).>> 

Not if prolacertiforms and Dave Peter's phylogeny are used.  

<<I guess what I`m looking for  as a common link between aves, and 
pterosaurs is as yet a hypothetical form. Still, I would think that what 
can be observed of the similarities between the two forms, even in their 
advanced, specialized states might suggest the existance of such a 
hypothetical ancestor.>>

I guess that you can then argue that tritylodontids are 
multituberculates, bats are related to birds and pterosaurs, etc.

<<I know that "hypothetical" arguments tend to be weak, and subject to 
flaw, but , again, in these areas of scant fossil evidence, what 
recourse is there?>>

Look at hypotheses with abundant fossil evidence?  :-) 

Lets do just that...  I think that maniraptorans ARE the ancestors of 
birds, but just for argument lets look at the main, most viable 
alternative to the maniraptoran/bird condition; the crocodylomorph 
hypothesis.

Morphological evidence in crocodilians for avian relationships:

1)  Fenestra pseudorotunda.  
2)  Superior tympanic diverticulum with corresponding recess.
3)  Posterior tympanic diverticulum  "           "                  " 
4)  Anterior tympanic recess crossed by carotid.  
5)  Siphonium between pneumatic quadrate and articular.
6)  Teeth with the crown seperated from an expanded root by a distinct 
waist.
7)  Replacement pit in the tooth root closed ventrally until late 
developement.  
8)  Root cementum.
9)  Implantation of teeth in a groove without development of interdental 
plates.  
10)  Quadrate cotylus near the base of the paraocciput.  
11)  Bipartite quadrate articulation.
12)  Anterior-medial origin of temporal musculature.  
13)  Similiar development of the sternum.  

Most of these are valid characters, though most all are found in 
theropods.  But look at how strong this origin of birds is compared to 
the pterosaurian ancestry of birds.  Now, if the main alternative to the 
maniraptoran/bird connection are this strong, you most have an 
alternative hypothesis that is at LEAST as strong as this one.  

Oh, and also.  If theropods are the ancestors of birds (and even if they 
are not), most of the characters above are CONVERGENCES between 
crocodylomorphs and maniraptoriform theropods.  

Now, pterosaurs are much too specialized to be avian ancestors with 
their procoelus vertebrae, elongate digit IV, uropatagium, 
brachioptagium, and specialized wrists.  They are also very primitive in 
antorbital and periotic pneumatization.

<<Now, I guess this will remain as another of my "hunches" until some 
solid fossil evidence shows up to prove, or disprove my hypothesis. 
Thanks for the "debate", it has given me much more to think over. And, I 
don`t want to get into the "maniraptoran ancestry of birds," or vice 
versa, as I`m sure that`s already been debated to the hilt!>>

It hasn't really been debated much on the list lately.  Everybody thinks 
that birds are dinosaurs :-)  Now, if we had any combination of Sam 
Tarsitano, Larry Martin, Max Hecht, Kenneth Whetstone, or Alan Feduccia, 
it will be debated more.  

Another weakness of your hypothesis is its interpretation of secondary 
flightlessness in maniraptoriforms.  A far better interpretation is that 
they are avian ancestors not avian descendents. 

Matt Troutman

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