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Re: Sci Am - present.



At 09:08 PM 2/13/98 -0000, J. Jackson wrote:

>I think the steady convergence 'TOWARDS' birds in other theropods, in the
>Cretaceous, is much more convincing, and necessitates, for me, the
>birds-to-theropods movement in the Cr.

What convergence towards birds during the Cretaceous (which, incidentally,
is abbreviated "K", not "Cr")?  With a few minor (toothlessness in Late K
ornithomimids vs. toothy Early K ornithomimosaurs) and a possible major
(presence of prefrontal in Deinonychus, absent in later dromaeosaurid)
cases, the later Cretaceous representatives of the various coelurosaur
lineages are no more or less "birdy" than the Early K repesentatives.  The
idea of a general change towards greater birdiness during the Cretaceous is
not based on fossil evidence.

(Parenthetically, this is much like so so-called "trend" towards forelimb
reduction in theropods.  Yes, it shows up in tyrannosaurids, carcharodontid
carnosaurs, and carnotaurine abelisaurids.  It also shows up in ratites,
flightless anseriforms, and the kakapu.  Does that mean Cenozoic birds trend
towards winglessness?)

>The Jurassic evidence either way is pretty well zilch.  Pro-BCF evidence
>that I know of?  Branched 'feathers' on Sino-type creatures if confirmed 
>(acquired presumably at least late J);

This is no more pro-BCF than it is pro-BAMM (birds are modified
maniraptorans).  Presence of some sort of proto-feathery itegument would be
necessary prior evolutionarily to bird flight, as feathers form the flight
surface.

>dare I say it - Protoavis;
>"wing-folding" in Herrarosaurus. 

"Wing-folding" in Herrerasaurus?  What exactly do you mean by this?  The
carpus of Herrerasaurus did not allow the maniraptoriform folding of the
forelimb.  Instead, Herrerasaurus has a much more typical, primitive state
of affairs.

>It's more down to overall feel at the moment I think,

Certainly describes BCF as currently formulated...

>and imagining scenarios with greater or lesser likelihoods.

And therein lies the problem.  These likelihoods are evaluated how, exactly?
By feel?  By dream?  By biophysical parameters?  By phylogenetic analysis?

>Possibly the main opposition to BCF arises from the evidence of (for me,
>bad) cladistics.  The obvious question now is "How can we tell how reliable
>blind cladistics is ?"  Darren Naish tells of an experimant in a field much
>richer in fossil evidence than dinosaurs - fish or shells or something. 
>There is a longish sequence where every single step, twist and turn of the
>way is known - but examples were taken at just a few intervals, to give a
>picture about as sparse as we get with dinosaurs.  I don't know the exact
>details, but would I have mentioned it if it hadn't been good news for BCF?
> Hopefully DN will fill us in.

Indeed.  Also, might I recommend a book on the subject of phylogenetic
analysis and fossils?  It is always good to get the details on how a
procedure works from those who use it, rather than primarily from those who
do not...

Smith, A.B.  1994.  Systematics and the fossil record: documenting
evolutionary patterns.  Blackwell Sci. Publications.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661