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Re: Air sacs in extant non-avian reptiles?



Eric Lurio wrote:
 
<That's because they both lack the singular trait that
makes tetropods, toes.>

  I would disagree with this definition, and if you
asked Per Ahlberg or Jennifer Clack, they would, too.
Tetrapods are distinguished by a great deal of cranial
features, more than pedal or manal. There are
non-tetrapods (eg., rhizodonts) that have incredibly
tetrapod-like limbs, not to mention structures
reminiscent of digits; acanthostegs and ichthyostegs
are polydactylous (greater than the "normal" five in
all { Lissamphibia + Amniota }) and I would assure you
not to make assumptions on limbs as being indicative
of phylogeny. Suggestive, yes, but not indicative.
There are easily ascessible materials on basal
tetrapods you can get on the net (Clack has a few
pages on the Tree of Life site that would be helpful
to look at) and in the more available literature, such
as _Nature_ and _Science_. *Eusthenopteron*,
*Eucritta*, *Ichthyostega*, and others are revising
previous notions of phylogeny and development of
certain "tetrapodan" features, and there was quite a
mosaic going on there.

<They also have two chambered hearts and gill slits
all their lives. The gills in amphibians, are external
and look and work differently than fishes.>

  Bah. Salientians (frogs, toads, fossil allies like
*Prosalirus*) lack gills or chondrosteous gill arches
as in salamanders in adulthood, and that doesn't make
frogs non-amphibian, and therefore amniotan. Don't
rely on a single feature to qualify phylogeny or
diagnosis of "status" of an animal.

  Ron Orenstein wrote:

<<The point is that in a phylogenetic sense, even if
we only consider living animals, a crocodile is more
closely related to a pigeon than to a gecko --
whatever their level of similarity. Therefore if
crocodiles are reptiles along with geckos, birds must
be too.>>

<But Birds have four chambered hearts and are
ectothermic. geckos do not and are not. Birds are
covered with feathers and geckos with scaly skin.>

  Means diddly-squat. There are hairless mammals [us]
that do not have a low-grade or poikilothermic
metabolism (as in naked molerats)

<Something clearly happened to birds to stop them from
being reptiles, just as the evolution of toes stopped
amphibians from being fish.>

  *Eusthenopteron* and allies have toes and are
neither amphibian nor "fish." They are something
in-between.

  What stops you from being an ape? A primate? What
quality would you have to lose that would remove you
from consideration from being a mammal (this is closer
to the level of separation you are arguing, but I
could say "would you have to loose the amnion sac in
your gestation to become a non-amniote tetrapod? Are
you therefore an amphibian?").

  Birds are what they are regardless of ancestry.
Arguing some antiquated, Scala Naturan state of
organization in relationships or degree of development
is not a scientific methodology. 

=====
Jaime "James" A. Headden

  Dinosaurs are horrible, terrible creatures! Even the
  fluffy ones, the snuggle-up-at-night-with ones. You think
  they're fun and sweet, but watch out for that stray tail
  spike! Down, gaston, down, boy! No, not on top of Momma!

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