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Re: Bird Origins + Ref:



> A fairly recent article suggests that they have an alternative to
>"dinosaurs as avian progenitors":
>
>Feducca, A & Wild, R (1993).  Birdlike Characters in the Triassic Archosaur
>>Megalancosaurus<.  Naturwissenschaften 80:564-66.
>
>The photo they have of this specimen is the most bird-like of Mesozoic
>animals I have seen (please correct me on this!).  The beak and scapula
>morphology is so much like a modern bird, it scares me.  I'm not a
>cladogram expert (and wish not to be ;-) so excuse what follows.
>
>Feducca & Bird show that the origin of aves can be supported by
>synapomorphies (homoplasious) from both the archosaur and dinosaurs.  They
>actually strongly suggest that theropods did NOT give rise to the birds,
>but are only "related" - I would like opinions on this, but remember I'm
>only stating what they have written ;-)
>
>Could this mean that birds (and flight - excluding pterosaurs at this
>moment) have evolved twice?  If so then which lineage gave rise to our
>modern birds - dinosaurs or archosaurs??????

Darren et al -

        Feduccia and Wild aren't the first ones to purport
_Megalancosaurus_ as the true bird ancestor; Sam Tarsitano has been doing
it for a while, based mostly on the presence of a (not "the")
semilunate-chaped carpal in the wrist, and the assertion that this single
carpal in _Megalancosaurus_ is the true homologous ancestor of the same
feature in birds, whereas the "semilunate" in theropods consists of two
fused carpals, as witnessed (in his paper) in the primitive wrist of
_Coelophysis_.

        Feduccia and Wild add to this argument using a large number of
skull characteristics, and a few postcranial ones.  We'll come back to the
skull in a sec; amongst the postcranial elements, they identify a large
V-shaped furcula (almost as large as the skull) and a strap-like scapula,
which are bird-like traits, certainly!  However, a  reanalysis  identified
the "furcula" to be two thoracic ribs.  Thus, the animal lacks a furcula;
it also, notably, lacks a sternum.

        Feduccia and Wild mention no other avian autapomorphies in the
postcrania, although they describe some highly generic features, such as a
"very flexible neck" and "bird-like proportions of the forelimbs."  These
are traits which are common in arboreal animals, and certainly developed
numerous times amongst many groups of animals.

        Back to the skull:  Yes, the skull of _Megalancosaurus_ looks a lot
like that of a bird, especially in the cute piece of propaganda Feduccia
and Wild use by placing the skull of a moorhen in an identical pose next to
the photo of a _Megalancosaurus_ specimen.  However, when one ceases for a
moment to use cladistics and steps back from that picture, one is forced to
think:  in the ancestry of birds, what features _should_ evolve first?  I
think that it should be clear that one would expect the skull to be amongst
the last things that would evolve, well after the rest of the body has
adapted for flight.  First we should see the clavicles and sterna appear,
possibly along with feathers.   The arm proportions would follow, and then
the fusion of the manual elements.  Along with the proportions of the
forelimb would come the shifting of the center of gravity by shortening the
trunk, and the fusion of the tail elements into a pygostyle.  Feet
adaptations might be next, although these aren't critical.  But one could
easily obtain a perfectly good bird with all these adaptations, and still
have a fairly reptilian skull.  In fact, this _is_ what we see, from
_Archaeopteryx_ to _Sinornis_, _Iberomesornis_, _Concornis_, through
_Ambiortus_, and onwards to more modern birds (I'm still highly skeptical
of claims that all these birds are enantiornithurans, and that no modern
bird fossils are known from the Mesozoic).  At least _Archaeopteryx_ and
_Sinornis_ retain rather reptilian skulls, complete with teeth.  They also
lack the "saddle-shaped" cervical vertebral articulations found in modern
birds, demonstrating that even the neck could have -- and did -- evolve(d)
later, _after_ the rest of the bird body did.  It isn't the skull that
makes a bird; it's the body!

        Thus, although _Megalancosaurus_ did indeed have a bird-like head,
it really couldn't have been a bird, or a bird ancestor.  (I suppose it's
possible that a number of the "bird-like" traits in the skull of
_Megalancosaurus_ are neotonic -- that is, they're there because the animal
is a juvenile, hence the large orbit and short snout, etc., but I don't
know this, not ever having seen the specimen).  We should also examine
traits that _Megalancosaurus_ has that birds don't, such as a tetradactyl
manus with short stubby fingers, a specialization for arboreal habits, and
not likely to be lost, methinks.  If birds evolved from this, they could,
and I think should, have retained a tetradactyl manus.  Why would they lose
a digit?  I have the same questions for the large, robust four metacarpals
in the hand of _Protoavis_, which I also don't think had anything to do
with birds.




Jerry D. Harris
Denver Museum of Natural History
2001 Colorado Blvd.
Denver, CO  80205
(303) 370-6403
Internet:  jdharris@teal.csn.net
CompuServe:  73132,3372

--)::)>   '''''''''''''/O\'''''''''''`  Jpq--   =o}\   w---^/^\^o

Overheard in the Denver Museum's
old Fossil Mammal Hall, from a mother
to her daugher:

"See there?  That's the camel-dinosaur, and
the horse-dinosaur, and the elephant-dinosaur..."

--)::)>   '''''''''''''/O\'''''''''''`  Jpq--   =o}\   w---^/^\^o