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Re: Turtles and Crocodylians are not Reptiles - no? What are they?



Turtles are the closest to archosaurs now?

I hadn't heard that...
As far as I know they are still out on grouping them with Anapsids or Diapsids, 
due to conflicting morphological vs molecular data.

Now you are grouping them firmly within Diapsids at a specific position? closer 
than lepidosaurs (snakes and lizards?) and squamates(tuataras)?

Now that you mention it... I seem to recall hearing about fish and mollusks 
that change sex with age or in response to environmental factors.

Perhaps specific sex chromosomes are an adaptation to being warm blooded? 
Temperature regulation would seem to pose problems with temperature related sex 
determination.
My limited knowledge of specific sex chromosomes extends mainly to mammals, and 
drosophilia flies, and limited with birds - 2 of 3 being warm blooded, and 
among vertebrates, thats 2 for 2.
But then.... if crocodiles had warm blooded ancestors - my "hypothesis" would 
predict their sex was genetically determined, not environmentally.

As for temperature controlling sex- doesn't it just skew the ratios? but both 
sexes are born for any given temp? 
There are XY "female" humans, where the Y chromosome didn't function properly 
in early development - temp based sex determination doesn't immediately rule 
out genetically determined sex - could temperature have an adverse effect on 
certain proteins/enzymes/receptors/hormones, etc during development, that skews 
the phenotypic ratio away from the genetic ratio?

Can anyone say definitively if crocs have hetero sex chromosomes? other 
reptiles? especially those displaying large amounts of sexual dimorphism?
What are the theories on how the XY mammals and WZ Aves evolved?


--- On Mon, 7/21/08, Roberto Takata <rmtakata@gmail.com> wrote:

> From: Roberto Takata <rmtakata@gmail.com>
> Subject: Re: Turtles and Crocodylians are not Reptiles - no? What are they?
> To: erikboehm07@yahoo.com
> Cc: dinosaur@usc.edu
> Date: Monday, July 21, 2008, 4:25 PM
> On Mon, Jul 21, 2008 at 7:54 PM, Erik Boehm
> <erikboehm07@yahoo.com> wrote:
> > On the subject of differentiating reptiles from
> mammals, but including birds with reptiles, I have to ask
> about one distinction between birds and mammals not touched
> upon yet.
> >
> > WZ vs XY sex chromosomes.
> >
> > Do all the extant members of the traditional reptile
> group have WZ sex chromosomes, where the heterozygote(WZ)
> is female, and the homozygote(ZZ) is male, in contrast to
> mammals (XY=male, XX= female).
> 
> Well, in turtles as well as in crocodilians, we have
> temperature
> dependent sex determination. Crocs are the extant closest
> relatives of
> birds - and probably turtles are the extant closest
> relatives of
> archosarus (birds + crocs).
> 
> []s,
> 
> Roberto Takata