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Message text written by INTERNET:jwoolf@erinet.com
>Is or isn't the perforate acetabulum diagnostic for _all_ Dinosauria?  If
it isn't,
why is it still in the list of diagnostic features?  If it is, then how can
you be
sure that ankylosaurs are dinosaurs, when they don't have all the
features for dinosaurs?  Are you assuming ankylosaurs are dinosaurs because
workers said they were dinosaurs?  Or are there perhaps other features that
more useful than the acetabulum for identifying something as a dinosaur?

Jon, I know you better than to think that you don't understand the concepts
of ancestral states and secondary reversals!  With primitive dinosaurs
known (_Eoraptor_, _Pisanosaurus_, etc.), we know that a diagnostic
characteristic of _primitive_ dinosaurs is a perforate acetabulum.  Thus,
any descendant of those basal dinosaurs is, by definition, a dinosaur.  If
we place ourselves in the Late Triassic, looking at those dinosaurs, we
understand that there is a forthcoming huge quantity of time in which
dinosaurs could evolve and diversify.  Diversification, of course, is the
result of evolution, which, in turn, is accompanied by morphological
change.  One of the potentials of a morphological change would be a closure
of the perforate acetabulum -- it wouldn't _necessarily_ happen, but it
        Along comes _Scutellosaurus_, or some similar organism, which has a
plethora of armor scutes on the body and some other autapomorphies which
allow us, several hundred million years later, to find it unique enough to
found the clade Thyreophora.  Throughout the Mesozoic fossil record, we
find other organisms which have the autapomorphies of the Thyreophora, and
thus, are thyreophorans.  But many of them, as you have noted, have lost
the perforate acetabulum.  Does this mean that they are dinosaurs?  Yes,
since their _ancestors_ were dinosaurs, the same way that, even though we
have lost lobe fins, we are still technically sarcopterygians, since our
ancestors are sarcopterygians.  Secondary reversals do not an exclusion
make.  After all, if one of your children happened, by virtue of some
mutation, not possess one of the diagnostic characteristics of _Homo
sapiens_, would you place it in its own species and/or genus, given that
you, as one of the parents, _is_ _Homo sapeins_?

           ____/_\,)                   ..  _   
--____-===(  _\/                        \\/ \-----_---__
           /\  '                        ^__/>/\____\--------

Jerry D. Harris                         (505) 841-2809
Fossil Preparation Lab                
New Mexico Museum of Natural History        
1801 Mountain Rd NW                           
Albuquerque  NM  87104-1375             102354.2222@compuserve.com