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RE: PTEROSAURS, PROTOBIRDS, AND CONVERGENCE



From: Matthew Troutman <m_troutman@hotmail.com>
To: dinosaur@usc.edu <dinosaur@usc.edu>; larryf@capital.net
<larryf@capital.net>
Cc: m_troutman@hotmail.com <m_troutman@hotmail.com>
Subject: Re: PTEROSAURS, PROTOBIRDS, AND CONVERGENCE
Date: Sunday, October 04, 1998 11:59 AM

<<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.

Chatterjee indicates the presence of a triosseal canal for the
supracoracoideus pulley  as well as a fairly large sternal keel for
attachment of the wing elevating muscles, in what he determines is the
earliest bird "Protoavis". That these flight features are not present in
Archaeopteryx, is interpreted by some to be an indication  of it`s tendency
towards a condition of secondary flightlessness, and hence it`s not being on
a "direct" line in the evolution of modern forms.

<<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.

Yes, there is a PRIMITIVE mesotarsal joint which is "similar" to the
Advanced Mesotarsal ankle joint but, (according to Spencer G. Lucas'
Dino-text  pg 49 ), ..."yet the two ankles are fundamentally different. In
the AM ankle the astragalus is much larger than the calcaneum and both bones
are rigidly attached to each other and to the tibia." Carroll also mentions
that in pterosaurs there is fusion of both these elements to the end of the
"crus" as in birds and many dinosaurs.

<<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.

Seems like another "bird-like" specialization explained away by
"convergence"!

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.

Maniraptorans, (as I see it,), split off from birds at a later date than
Thecodonts.

  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.

I`m not sure I follow. (I`ll try reading this a few more times).

<<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 why not in all the mammals?, unless it`s because not ALL of the
mammals were descended from bats.

<<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.

Depends on who`s constructing the cladogram, and their interpretation of
what constitutes a "secondary condition of flightlessness". (I imagine!).

Matt Troutman

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Larry Febo.