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Re: A bit about BCF theory (Lots-o-questions)
In a message dated 95-11-24 02:10:10 EST, Dinogeorge@aol.com writes:
>This compromise stablized for many millions of years (until the
>wing became well enough developed in the archosaur-to-bird
>lineage that its grasping function could be abandoned
>completely), because theropod after theropod shows up in the
>fossil record with the tridactyl manus. Then, all of a sudden, we
>have didactyl manus (tyrannosaurids), monodactyl manus
>(_Mononykus_), and of course volant birds, in which the three
>digits are fused into a unit. It is as if some kind of
>evolutionary threshold were crossed sometime in the Late
>Jurassic, as enough improvements occurred to the wings to allow
>the animals to forego grasping hands entirely.
I'm a little fuzzy on this part.
First, does this mean that theropods in general are offshoots from the main
line of archosaur-to-bird evolution? Or did they split off early, making
theropods a sister group to birds?
Since I don't know the answer to this question, I have more questions,
depending on the answer...
**CASE I : Theropods ARE offshoots of the archosaur-to-bird line**
Are tyrannosaurs, then, considered to be more bird-like than, say, the
dromaeosaurs, because the tyrannosaurs are further along the
archosaur-to-bird line, thus should be offshoots from a more "advanced" form
If THIS is the case, why should tyrannosaurs *lose* an extra finger, making
them didactyl, rather than having, say, the caudal-most finger (sorry, I'm
not sure which digit numbers tyrannosaurs retained...) be the result of a
fusion of two fingers, since that seems to be the trend in building a better
bird - lose digits IV and V, then fuse the rest together. Or did
tyrannosaurs, in fact, have a fused digit? If they did actually *lose* the
digit, rather than fusing two together, than what is wrong with the BADD - or
BAAM, if you prefer :o) - idea of "Well, what the heck, why _not_ lose a
couple of fingers?" How does BCF deal with this?
Do we see more "advanced" theropods having more-and-more restricted arm
movements, closer to the flight-stroke of volant birds?
**CASE II: Theropods branched off of the archosaur-to-bird line at some
point, then continued evolving separately from birds**
Why should theropods (e.g., tyrannosaurs, _Mononykus_) mimic the reduction of
fingers (e.g., tyrannosaurs, _Mononykus_) found in the bird line?
**IN EITHER CASE**
What exactly does the BCF version of the theropod-bird family tree look like?
Thanks for any clarification, and sorry for so many questions :o)
(What's that bit about "inquiring minds..."?)