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Re: PTEROSAURS, PROTOBIRDS, AND CONVERGENCE



<<I still say, out of all three cases, the structure of birds and 
pterosaurs are much, much, MUCH more closely allied morphologically! 
(did I say much too much?)>>

But the problem is; pterosaur shoulders are not similiar to bird 
shoulders in detail.  True, a supracoracoideus wing elevator may have 
been present, but the detail (no readily apparent triosseal canal, no 
bicipital crest) is very different.  Plus, early birds do not have a 
supracoracoideus wing elevator to the extent seen in pterosaurs and 
modern birds.  

<<I DO think that prolacertiforms are the best candidates for pterosaur 
ancestry, (although I personally would choose one that had clavicles 
intact), but it still is not visible in them the extended fourth wing 
digit that one would expect in a pre- pterosaur form, and therein lies 
the " gap". There might have developed a more "gracile form", with the 
extended membrane, retaining clavicles (furcula), that became adjusted 
to living  in the narrow branched world of gymnosperms, and developing 
angiosperm trees.>>

_Cosesaurus_, Peter's pterosaur ancestor (actually on step away from 
_Sharovipteryx_), has been considered a protobird, most recently by 
Larry Martin who thinks its a crocodylomorph.  However, _Cosesaurus_, 
and the prolacertiforms, are not even archosaurs.  

<<I don`t think that the similarities I am pointing out are all that 
distant, ie. The specialized redirection of the supracoracoideus tendon 
to act as wing elevator,  hollow bones,(whose ONLY function that I can 
think of would be as a flight adaptation), and there`s another feature I 
left out, that actually makes for a pretty strong argument in my favor 
(I think), that is .....the Advanced Mesotarsal ankle joint present only 
in Dinosaurs, Birds and (whadda ya know!) Pterosaurs....>>

Read above for the supracoracoideus.  The mesotarsal ankle joint is 
primitive for archosauromorphs.  Look at Peter's evidence, 
prolacertiforms have a mesotarsal joint.   

<<So,...we`re agreed that pterosaurs are derived from prolacertiforms 
that had no AM ankle joint.>>

Read above. 

<<My theory would propose that this condition first developed in the 
pterosaur line, and it would explain the passing on of this primary 
diagnostic condition to the group aves, and hence to their descendants 
the dinosaurs. Padian (I think) tries to show sisterhood of the 
pterosaurs to the Dinosauria by claiming them to be descendants of 
Lagosuchus (or similar), again implying a (to me) weak ground upwards 
evolution of the flight mechanism. Of course, if you agree with  Dave 
Peter`s phylogeny (or Wild`s ?), I guess then you`re calling the AM 
ankle in Pterosaurs just another example of Convergence???>>

Advanced mesotarsal joints are very easy to evolve in parallel since 
mesotarsal joints are primitive for many reptiles.  

What is so wrong with convergence?  Convergence has happened so many 
times in evolution.  If you cannot except convergence as happening in 
vertebrate evolution then I guess that tetrapods are lungfish, red 
pandas are related to giant pandas, whales are icthyosaurs, pterosaurs 
are bats, bats are monophyletic (I know, this is rather different from 
the other cases), mesonycids are carnivorans, hippos, elephants, 
sirenians, and rhinos are monophyletic, marsupials are monotremes, 
monotremes are not mammals, etc. etc. etc. 

<<My larger view of  Diapsid evolution is that the whole line 
essentially developed  within an arboreal enviornment, with a constant 
periodic sending of descendants back to ground level to lead a 
terrestrial, (or in some cases aquatic) existance. So, in my view, all 
of the Thecodonts, came from such prior arboreal ancestors, some no 
doubt from the direct line leading towards the avian group, therefor, 
they would be expected to have many similarities,...and NOT by 
convergence.>>

Lots of reversals are necessary for this to be true.  'Thecodonts' do 
not show any of the characters linking maniraptorans to birds or 
crocodilians to birds.  I guess that 'thecodonts' are convergent on the 
primitive condition then :-)  The best interpretation is that the 
characters linking the theropods to birds and corocs to birds are 
convergences between crocs and theropods (unless they turn out to be 
sister-group as suggested by Tarsitano) and crocs are convergent to 
birds and theropods since most evidence suggests that birds are 
theropods, tetanurines, avetheropods, coelurosaurs, maniraptoriforms, 
maniraptorans etc.

<<Convergence seems (to me) to be an overused, if not abused concept. 
One thing I learned in reading Feduccia`s book, is that convergence (or 
not) is sometimes very hard to detect.>>

Yes, but there still are cases where convergence can be detected like 
the cases above.  Converegences can be found many ways: through 
biomechanics, cladistics, embryology, etc.  The best way to find 
convergence in fossils is through cladistics.

<<Still, I cannot see how he fails to see the dino-bird relationship to 
be what it is, that is very close, actually one and the same  with 
theropods. So, how much better are many of you who see this close 
theropod-bird relation, and yet dismiss the other "birdlike" characters 
of the remaining sauropods ,and the ornithischians even, as 
being..."Convergence"???>>

Feduccia is looking at the same evidence and coming to differing 
interpretations, which is probably the biggest part of science :-)  
Looking at, and interpreting the evidence differently is how Tarsitano 
thinks _Megalancosaurus_ is an avian ancestor, how Whetstone and Martin 
find that crocs are closest to birds, and even how Brian Gardiner finds 
birds to be most closely related to mammals.  

<<What enviornmental force is acting to make these dinos appear 
"Bird-like", if in fact they weren`t directly descended from the bird 
line at some point in time??>>

Similiar functions?  :-)  Listen, from the best that we can tell, crocs 
are similiar to be in many features.  Follow?  Now, theropods are 
similiar to be birds IN MANY OF THE SAME FEATURES.  Follow?  Now, from 
our current knowledge of archosaurian relations (which can change but 
seems pretty solid) the closest relatives of crocs and theropods (I'm 
using theropods since the animals that lead to maniraptorans show many 
of the same features) DO NOT, that is DO NOT, show the same features.  
Now, what do we conclude.  That there are lots and lots and lots of 
reversals (which can be convergences on the primitive condition) within 
the groups leading to crocs and theropods?  Or, that some of the 
features that link both crocs to birds and theropods to birds are 
convergent between crocs and theropods, and since most evidence shows 
that theropods are more similiar to birds then crocs are CONVERGENT on 
theropods+birds.  This is how you find convergences on a cladogram.  

<<Someone PLEASE tell me why they developed hollow bones...many of them 
in seeming anticipation of flight (as this flies in the face of 
evolutionary theory as I know it!). (Oh boy, I didn`t want to start up 
the flames of BCF vs. BADD, but here it goes!).>>

Hollow bones are found in many mammals too.  The flight features you 
describe are also found in bats, so I guess that they are members of the 
bird+pterosaur clade :-) 

<<Yeah, but they had long enough to " specialize",(or not),  after my 
proposed pterosaur-bird split in the early Triassic...no?>>

Maybe not.  

>Well, I guess that last statement is a matter of opinion,...and 
certainly
>not mine!

Secondary flightlessness must be found on a cladogram or something 
pretty darn close.  

Matt Troutman

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