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RE: Air sacs in extant non-avian reptiles?



> In a message dated 7/31/00 8:08:02 AM, overmind@iobox.fi writes:
>
> <<
>  Dear all
>
> I have read that ait sacs extending from the lungs are present in some
> extant, non-avian reptiles. I'd be interested to know the names of these
> reptiles. Also, if someone has tried to explain the function of these
> structures in these dead-end lungs, please tell me.
>
> Best wishes,
>
> Henri Rönkkö >>

Dear Henri Rönkkö,

Please ignore the outburst of Eric Lurio, quoted below.  He seems to be very
emotionally attached to a number of out-moded concepts.

In point of fact, the majority of evolutionary biologists working on
vertebrate relationships today (paleontologist and neontologist alike) agree
that the evidence for the dinosaurian origin of birds is vastly stronger
than the alternatives.  A small group of physiologists and a couple of
paleontologists reject this idea quite loudly, although not very
effectively.  Lurio seems to have accpeted their arguments hook, line, and
sinker.

Theoretically independant of this is a shift in the convention of naming
groups, with a traditional system based on grades of organization and the
current system based on nested pattern of ancestry.  In point of fact, one
could simultaneously reject the dinosaurian origin of birds but still accept
birds as part of Reptilia (as they clearly are a member of the clade of
organisms descending from the most recent common ancestor of turtles,
lepidosaurs, and archosaurs (being archosaurs themselves!!).  In practice,
though, those people who reject the dinosaurian origin of birds also reject
both cladistic taxonomic conventions and cladistic phylogenetic
reconstruction methods.

Now, on to your real question:
There are various types of air sacs in birds (some in the skull, some in the
body).  With your reference to the lungs, you are referring to the ones in
the body.

The actual soft tissue of the air sacs have only been described so far in
modern birds. Indeed, we don't have them preserved in fossil birds of the
Mesozoic or Cenozoic, but their presence of some of them can be inferred by
complex cavities in the bone.  Chambers of the same morphology are found in
almost every advanced theropod dinosaur: dromaeosaurs, troodontids,
oviraptorosaurs, therizinosauroids, tyrannosaurids, ornithomimosaurs, and
carnosaurs.  Less complex chambers are found in ceratosaurian theropods and
sauropod dinosaurs.  Pterosaurs have similar chambers in their vertebrae.

Air sac tissue itself is not yet described for these fossils: only the bone
is found.  However, the presence of the airsacs is inferred for these
animals in the same way it is inferred for _Archaeopteryx_ or _Ichthyornis_
or other animals which are considered "birds" but which lie outside the
group comprised of all living birds (the only animals for which we can
definitely point to the sacs).  The histology of the bone surround these
chambers is identical to those seem in the pneumatic bones of birds.

As for the purpose of these air sacs: by shunting air from trachea to lungs
to air sacs and out again, there is a one-way (rather than two-way) flow of
air into the lungs.

P.S.  Although the position of birds as maniraptoran coleurosaurian theropod
saurischian dinosaurs lends support to the hypothesis that the chambered
vertebrae of saurischians were related to air sacs, some of the great old
traditional paleontologists, such as Swinton and Romer and Janensch (who did
not accept a dinosaurian origin of birds) also supported the idea of an
avian air sac system in Saurischia.

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> ELurio@aol.com
>
> Here we go again. There is no such thing as an avian reptile, so the term
> non-avian reptile is redundant like "non-photosynthisizing frog."
> I know that
> a tiny group of paelentoligists want to exert their power by
> destroying the
> class status of Aves by the power of pure fiat, but it's not
> going to work.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843