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



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).

So far I've only read about birds, mammals, and various arthropods (who are so 
distantly related to us I'll ignore them for now).

What is the condition found in the rest of the vertebrates excluding amniotes, 
ie fish and amphibians?

What would the transitional form be? how would such a division occur so long 
after sexual reproduction was already well established with well differentiated 
sexes?

Did some females "evolve into males" while the ancestral type males went 
extinct? 

Some creative chromosome rearangement/fusion?

What is the current leading model for when this change occured, and how?


--- On Sun, 7/20/08, Tim Williams <twilliams_alpha@hotmail.com> wrote:

> From: Tim Williams <twilliams_alpha@hotmail.com>
> Subject: RE: Turtles and Crocodylians are not Reptiles - no? What are they?
> To: dinosaur@usc.edu, erikboehm07@yahoo.com, twilliams_alpha@hotmail.com
> Date: Sunday, July 20, 2008, 11:22 PM
> Erik Boehm wrote:
> 
> 
> > Its impossible to form a monophyletic definition,
> there is no way to reconcile the exclusion of birds, or the
> > inclusion of synapsids but not mammals.
> 
> 
> True.  The issues are summarized here....
> 
> Modesto, S., and Anderson, J. (2004).  The phylogenetic
> definition of Reptilia.  Systematic Biology 53: 815-821.
> 
> They arrive at a stem-based definition for Reptilia (the
> most inclusive clade containing _Lacerta agilis_ and
> _Crocodylus niloticus_, but not _Homo sapiens_).  However,
> this revised Reptilia consciously excludes turtles from the
> definition, because of their uncertain phylogenetic position
> in amniote phylogeny.  Personally, I think the definition of
> Gauthier et al. (1988) is better, because it uses turtles,
> as well as squamates and crocs, as reference taxa.  So
> turtles, crocs and squamates would always be reptiles, and
> birds would fall inside Reptilia, whereas mammals (and all
> other synapsids) would fall outside.  
> 
> 
> > But a paraphyletic definition is pretty easy.
> 
> 
> Paraphyletic definitions are not standard practice in
> phylogenetic taxonomy, and I think they're probably a
> bad idea on the whole.  
> 
> 
> > Wouldn't this be a good start for a paraphyletic
> definition:
> > All aminotes excluding those closer to birds than to
> crocodiles, and excluding all those descended from the 
> > cynodonts
> 
> 
> >From a historical/typological perspective, birds fall
> more comfortably inside Reptilia than mammals given that
> they are descended from scaly ancestors (and retain scales
> on the feet, etc).  On the other hand, mammals do not have
> true scales, and never did.  Our skin has always been
> glandular, more like that of amphibians than the reptiles
> around today.  As thrashed out in a recent thread, the term
> "mammal-like reptile" is a misnomer.
> 
> Every phylogeny I have seen recovers the Synapsida
> (including mammals) as diverging from the lineage that gave
> rise to all other extant amniotes (crocs, birds, turtles,
> lizards, snakes, tuatara) and a host of extinct reptilian
> groups (plesiosaurs, ichthyosaurs, and the like). 
> Accordingly, new (phylogenetic) definitions of Reptilia
> include extant reptiles and birds, but not mammals.  
> 
> BTW, as a clade descended from cynodonts, mammals are
> themselves cynodonts.   In other words, Mammalia is a clade
> inside Cynodontia.
> 
> 
> Cheers
> 
> Tim
> 
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