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

> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] 
> On Behalf Of evelyn sobielski
> > 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).

In fact, we have no compelling evidence that enantiornithines or
Patagopteryx had WZ sex determination (or, alternatively, that Allosaurus
lacked it). This is simply not something that fossilizes.

And, consequently, why it would utterly fail as a useful criterion for
boundary conditions of "Reptilia" if you are interested in fossil

Thomas R. Holtz, Jr.
Email: tholtz@umd.edu   Phone: 301-405-4084
Office: Centreville 1216                        
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
Fax: 301-314-9661               

Faculty Director, Earth, Life & Time Program, College Park Scholars
Fax: 301-405-0796

Mailing Address:        Thomas R. Holtz, Jr.
                        Department of Geology
                        Building 237, Room 1117
                        University of Maryland
                        College Park, MD 20742 USA