CONVERGENCE HAPPENS

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

<<"Seeley showed that these holes in the bones of pterosaurs were not 
merely air spaces, but pneumatic foramina, structurally inseparable from 
those of birds;and he drew the (then) logical inference that they were 
for admission of respiratory surface into the limb bones...." (K. 
Padian-"A functional analysis of flying and walking in pterosaurs" 
Paleobiology 1983)>>

I'll have to go check on this one.  I could be wrong, even though it 
does not matter much.  I was did say that the elongate fourth digit is 
pneumatized, though.  Regardless of whether pterosaurs had this system 
or not, all basal birds, including it seems, most enantiornithines lack 
this type of pneumatization which renders your argument null.  

<<<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.>>>

<<Even among modern birds, this feature is exhibited to less extent in 
some groups,...(could be a secondary loss of function).>>

It is seen in most all birds except for anhimids and some ratites if 
memory serves.  This still does not mean much for pneumatopores in the 
limbs seem not to be present until ornithurine birds.  The only report 
of a pneumatic foramen in a limb bone of a basal bird is that 
_Confuciusornis_, but this feature seems to be manufactured.  

<<I`m a bit vague on how comparisons are made via braincase structure, 
or what about behavior can be determined from such evidence. I`ll just 
reiterate the position of Tilly Edinger on this subject (as quoted from 
Wellnhofer`s "Prehistoric Flying Reptiles"). "Tilly Edinger`s research 
showed that, even in the upper Jurassic, pterosaurs had developed brains 
that were more like those of birds than the brain of their contemporary, 
the 'primordial bird' Archaeopteryx. Thus the pterosaur brain was by no 
means reptile-like and small, as in modern crocodyles and lizards, but 
closer to that of a bird in shape and size. This was an important 
prerequisite for flight control and steering.">>

This is a function related similiarity and one that is not really that 
great in the first place.  Pterosaur EQ is really lower than most 
theropod dinosaurs.

<<<Hell, even Haematothermia has more osteological characters of greater 
weight than these.>>>

<<There are other structural characteristics in common involving posture 
and the advanced mesotarsal ankle joint (which I believe only evolved 
once). These I didn`t mention because it could be argued that they were 
primative to the Theropod group that allegedly gave rise to both birds 
and pterosaurs, ..except that I believe it started in the Pterosaurs and 
was passed down to both avian and theropod descendants.>>

Haematothermia, the bird-mammal clade, as defined by Gardiner shares 
many osteological characters such as a supra-acetabular shelf, 
thecodonty, and cnemial crests as well as some soft-anatomical features.  
Some of these are pretty robust.  Pretty big convergences?

You see, I'm not supporting Haematothermia, I'm just showing you that 
even the most remote hypotheses regarding avian origins (and this one 
even accepts that there is a sequence from basal theropods to birds) are 
more robust than the pterosaur-bird hypothesis.  Just because it 
accounts for convergence in a few characters does not mean that it is 
any stronger than any other hypothesis.  

<<If you need to view even more characters that show the closeness 
between Pterosaurs and Birds, you can check out K. Padian`s paper "The 
Origen of Pterosaurs" (1984), where even he makes the statment..."The 
detail of many resemblances between pterosaurs and modern birds is 
uncanny..."  (Although I don`t doubt that from his position, he would 
argue that what couldn`t be derived from early theropods, would have to 
be a convergent development of some sort, and in this I wouldn`t 
agree)>>

All flying animals and some gliding animals share several structural 
similiarities: a strut (either clavicle or coracoid) in the pectoral 
girdle, elongation of elements of the forelimb including the strut bone, 
a scapula that this parallel to the vertebral column, a broad sternum, 
etc.  

<<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`m sure Feduccia would agree....He`d say that all "avian" traits in
Dinosaurs are convergent! Seems to me that we had better come up with 
definative answers as to what exactly constitutes "Convergence", so that 
it`s not used as a "catchbasin" for whatever we cannot fit into our 
Phylogenetic sequences!>>

You're skirting the issue.  Are you proposing a complete overhaul of 
what we know about squamate phylogeny and evolutionary trends?  Putting 
amphisbaenians inside Platynoa would require many reversals and 
convergences.  Listen and listen well, convergence happens and sometimes 
it is very strong.  Gaviiforms, podicepiforms and hesperornithids share 
many features realted to swimming, and some have argued for monophyly of 
these three groups, but in retrospect these features are probably not 
homologous because of the subtle differences in knee structure, swimming 
style, and shared derived features with other birds.  If we try to 
account for all convergence that happens in all our accepted 
phylogenies, we end up with birds and mammals as sister-groups, dipnoans 
and tetrapods as sister-groups, lycopsids as the closest group to seed 
plants, crocodylomorphs and birds as sister-groups, cetaceans as 
ichthyosaurs, sauropterygians and ichthyosaurs as sister-groups, bats as 
pterosaurs, pterosaurs as mammals, etc. etc. etc.  

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

<<Archaeopteryx also had no keeled sternum, and only some seemed to have 
a furcula, this could mean that Archie was on the road to becoming 
secondarily flightless....others have said as much.>>

The lack of furcula in some _Archaeopteryx_ specimens is likely a 
preservational or ontogenetic characters.  _Archaeopteryx_ is obviously 
a flying creature and I can find few characters that indicate that it is 
on the way to becoming secondarily flightless.  For your logic to work, 
ALL basal birds were on the way to becoming secodarily flightless.  A 
more parsimonous interpretation is that we are actually witnessing the 
transformation of the coracoid from a slightly specialized state in 
dromaeosaurs to the modern avian coracoid.  

Read that Tarsitano paper carefully.  

Matt Troutman 
m_troutman@hotmail.com


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