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Re: Archosaur Origins...was:MESENOSAURUS ERRATA.

-----Original Message-----
From: Dinogeorge@aol.com <Dinogeorge@aol.com>
To: dinosaur@usc.edu <dinosaur@usc.edu>
Date: Thursday, August 23, 2001 2:51 PM
Subject: Re: Archosaur Origins...was:MESENOSAURUS ERRATA.

>>I wrote:
>> I`m curious,...are you implying that since birds have a prolacertilian
>> ancesty, they might be considered close sister group to pterosaurs?

>This has quite a bit to do with taxonomy, so let me get into that a bit.
>in the old days, before fossils were incorporated into taxonomy, the
>system was essentially a crown-group classification. Evolution had not yet
>been conceived of, and there was no plethora of fossil taxa that needed to
>classified. Crown groups are pretty easy to define by morphology and
>typology, because the stem organisms intermediate between crown groups--the
>organisms that don't have all of the type features of the crown groups--are
>extinct (that's what being a crown group is mainly about), and that's how
>taxonomy was done then.

Perhaps I should have been more careful in definining my "terms". I suppose
most people think of birds as the modern form. I always assume they include
the stem group stretching back (I believe) into the Triassic, and would be
defined as a true flying form possessing flight feathers (of some kind).

At any rate, I was curious as to how close to pterosaurs you think these
Triassic forms might have been? Assuming that  both evolved from
prolacertiforms, and within a fairly narrow time frame of the late Triassic
(if you count Protoavis as an example), I would think that they might have
had a common prolacertiform ancestor. Do you agree?

And for that matter, if "birds" did get their start from the prolacertiform
group, and theropod dinos were their flightless descendants, dosen`t this
imply that the other dino groups (Sauropod and Ornithischian) also had
prolacertiform ancestry???