Newsgroups: sci.bio
Path: utzoo!rising
From: rising@utzoo.uucp (Jim Rising)
Subject: Closest relatives of birds
Message-ID: <1989Nov27.150136.17007@utzoo.uucp>
Organization: U of Toronto Zoology
Date: Mon, 27 Nov 89 15:01:36 GMT

It is probably correct to say that the closest relatives of birds
are dinosaurs, but the alligators are also close relatives of 
dinosaurs--and they do grow continuously.  Basically, birds,
dinosaurs, and crocodiles are in the subclass Archosauria.
The crocs. are in the Order Crocodilia, and the birds are apparently
derived from animals that would have been classified in the 
Order Saurischia, Suborder Theropoda--the bipedal, carnivorous
dinosaurs, like Tyrannosaurus rex (actually more like Compsognathus,
for those of you who know it).  Thus, if the Reptilia were a 
"monophyletic" group in the sense of Hennig the birds would be 
included--and probably the mammals as well.

Contrary to what is apparently a popular belief, the above arrangement
is NOT a new one, based on fancy new molecular data (although they
support the relatively close arrangement of crocodiles and birds--havn't
figured out a way to look at dinosaurs in this way yet), or on insights
from cladistics, but rather was proposed in the 19th Century by T.H.
Huxley--and has been widely accepted by comparative anatomists since.

Concerning hair on pterosaurs, that is apparently possibly an artefact
of preservation.
In any event, it probably would not be homologous to hair on mammals,
and feather of birds are indeed very like scutes (not scales) on 
"reptiles"--more like scutes than hair is.  Lots of animals have hair-
like structures that are not mammalian hair.
This is not intended as a criticism of the suggestion that dinosaurs
may have shown definitive growth.  I suspect this isn't known, and may be
unknowable--but certainly isn't something that I can comment on.

--Jim Rising
-- 
Name:     Jim Rising
Mail:     Dept. Zoology, Univ. Toronto, Toronto, Ontario, Canada    M5S 1A1
UUCP:     uunet!attcan!utzoo!rising 
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