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Re: Synapsids weren't reptiles?
--- On Mon, 6/30/08, T. Michael Keesey <email@example.com> wrote:
> > And I know- some mammals excrete uric acid too, like
> kangaroo rats?
> I think all do, actually, but not in a concentrated form
> like crown
> sauropsids. Actually, I'm not really clear on this, so
> someone more
> knowledgeable should feel free to step in. And don't
> turtles differ
> somehow from other crown sauropsids in this respect?
Unless I missed a recent discovery, Kangaroo rats excrete urea just like all
other mammals. They are unique in having incredibly long loops of Henle, which
allows for the urine to be extremely concentrated, but other than that, I
believe all mammals are ureotelic (though uric acid does get shot out in the
urine, it is not the primary means of removing nitrogenous waste).
Most sauropsids are uricotelic; concentrating all nitrogenous waste in the form
of uric acid crystals. However, as alluded to with turtles (and even then it
depends on what turtle species you are talking about), uric acid can be shot
out in a manner more like urine, as long as there is enough available water to
go around. This leads to hybrid classifications like ureo-uricotelic, or
ammono-ureotelic in the case of _Trionyx_ turtles.
HP Adam Britton, from the archives, mentioned how available water affects urate
production in _Crocodylus porosus_ in estuarine and freshwater environments.
Of course, since nitrogen catabolism/excretion is so dependent on water
availability, I am left wondering if whales and pinnipeds might have evolved
the ability to simply excrete ammonia, like so many marine organisms do.