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

> What are the theories on how the XY mammals and WZ Aves
> evolved?

As regards WZ, unfortunately all living birds are WZ. No platypus extravaganza 
here. Given that crocs are heavily modificational, WZ must have evolved 
somewhere in the dino lineage. But where? Probably in theropods or in any case 
rather late; the W and Z chromosomes of ratites are still rather similar (you 
need FISH or such to tell them apart; it's not reliably possible in a 
karyotype).


Regards,

Eike


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