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Re: Sci Am - present.



At 02:12 PM 2/18/98 -0500, Michael Teuton wrote:
>
>> 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.
>> 
>I read this as I have other statements which I have generally agreed
>with without much thought.  However, I went back to it today and reread
>it a few times.
>
>Why would it be necessary for dinosaurs to develop feathers prior to
>flight?  Their cousins may have flown without them and bats do now. 
>Would it be possible for dinosaurs to have developed flight then
>developed feathers?

Note: in my statement above I clearly (and by choice) wrote "proto-feathery
integument", NOT feathers.  It may well be that true, complex, modern style
feathers developed only after the pre-existing integument was already being
used as a gliding surface.

>I'm familiar with the arguments about endothermy vs
>ectothermy and powered flight. Is there room for argument for limited
>powered, gliding and thermal flight in featherless dinosaurs?

Actually, no, their isn't much support for a stage in which dinosaurs could
glide on skin-based patagia.

The anatomy of theropods in general, and birds in particular, differs from
all the typical gliders and fliers in that the hand, although long, is VERY
compact.  There does not seem to be much of a surface over which skin could
be strecthed.  This differs from pterosaurs (which minimally had a patagium
from hips to underarm, down the arm to the finger tip, and back again) and
bats (which have GREATLY expanded digits overwhich to connect the skin).

Bird flight doesn't work without featers already being present: they are the
flight surface over which air must pass.  Their are no anatomical signatures
for patagia connecting body and arms at any point in bird history.  However,
feather-like structures had long been suspected, and now confirmed, in
nonavian theropods.

Hypothesizing a stage in which protobirds used a flight surface developed
out of elongate integumentary structures requires only intermediate steps
between observed character states in fossils.  Hypothesizing featherless
skin patagia in protobirds requires structures not observed in either the
general group (nonavian theropods) or the derived group  (birds).

So, is it possible that there was a dermopteran- or flying squirrel-like
stage in bird evolution?  Sure, it's possible, but there is no evidence to
support it, and there is evidence to support an alternative scenario.

Note, however, that none of the above requires particular scenarios of
thermal physiology.  It only requires the presence of integumentary
structures already observed in nonavian theropods, and of feathers already
observed in birds. 

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