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In a message dated 96-09-30 12:01:57 EDT, Dinogeorge@AOL.COM writes:

>> If arboreal animals cannot develop an erect posture, then just how
>> >did< monkeys do it? Ditto kangaroos, whose present-day smaller,
>> arboreal relatives are pretty much erect.
> Your comments about acquiring a fully erect gait address a point that I've
> been pondering for some time myself. One way to overcome this little
> difficulty is to suggest that dino-birds (like other thecodontians) were
> already semi-erect quadrupeds when they became arboreal. This also makes it
> easier to understand where phytodinosaurs came from and why most of them
> remained quadrupedal.

Actually, I forgot part of my own theory here (see my article in OMNI).
Evolution of the semi-erect and fully erect stance had little if anything to
do with flight or arboreality, but more to do with the decoupling of
respiration from locomotion and with near-completion of the development of a
four-chambered heart. Arboreal archosaurs could well have had a semi-erect
stance or even a fully erect stance. The works to read about the details are
by Cowen and Cowen & Lipps:

Cowen, R., 1994. History of Life, 2nd edition, Blackwell Scientific
Publications, Cambridge, Massachusetts.

Cowen, R. & Lipps, J., 1982. "An adaptive scenario for the origin of birds
and of flight in birds," Third North American Paleontological Convention,
Proceedings 1: 109:112.