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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: email@example.com Phone: 301-405-4084
Office: Centreville 1216
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
Faculty Director, Earth, Life & Time Program, College Park Scholars
Mailing Address: Thomas R. Holtz, Jr.
Department of Geology
Building 237, Room 1117
University of Maryland
College Park, MD 20742 USA