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Re: ACROCORACOIDS AND REPTILE PHYLOGENY



-----Original Message-----
From: Matthew Troutman <m_troutman@hotmail.com>
To: larryf@capital.net <larryf@capital.net>; dinosaur@usc.edu
<dinosaur@usc.edu>
Date: Sunday, February 28, 1999 4:32 PM
Subject: Re: ACROCORACOIDS AND REPTILE PHYLOGENY


(snip)


>Anyway, on to acrocoracoids...
>
>In the other reply to my previous post, Larry said that the presense and
>elevation of the acrocoracoid process is a sure way to tell whether or
>not various maniraptoriforms are secondarily flightless.  Fair enough,
>although I guess that _Sphenosuchus_, which is one step away from being
>a crocodile, is secondarily flightless!  The _Sphenosuchus_ coracoid not
>only shows a biceps tubercle in the theropod position, the coracoid is
>elongate in the manner of the various maniraptoriforms.  Walker (1972)
>regarded the shoulder of _Sphenosuchus_ to be very birdlike, possibly
>more so than _Archaeopteryx_!
>
>Secondarily flightless sphenosuchids?  Keep it coming...
>
>Matt Troutman
>m_troutman@hotmail.com
>


Yes, I did say that the condition in theropods would be an indication of
secondary flightlessness. I think the acrocoracoid process began with the
Pterosaurs, and of course there would be intermediate steps leading up to it
that would also show development of the biceps tubercle. I don`t know the
exact path to the development of this redirection of the supracoracoideus
tendon over a "pulley-like" structure (the acrocoracoid processs), but
suspect it had something to do with improving the gliding stage that flight
required. Probably had to do with manuverability for more precise landings.
Perhaps there were gliders among other archosauromorph groups that gave rise
to Sphenosuchians and other such "Thecodonts". I personally think that all
of the Diapsid line started in an arboreal enviornment (to escape
competition with ground-dwelling Synapsids). Some were climbers, some , no
doubt, tried gliding. Many gave rise to various larger descendants that make
up the mixed bag of Thecodonts, at various stages. Any of these might have
inherited the biceps tubercle from a gliding ancestor.