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Re: Segregated vs age-mixed sauropod herds



David Marjanovic <david.marjanovic@gmx.at> wrote:


> In terms of
> continuity with the paleontological literature, the
> crown-group definition for Mammalia (1988) was a
> considerably worse idea than the *Sinoconodon*-+-crown-group
> definition which was published in 2002 or 2004.


If Mammalia is indeed defined to be the crown clade ONLY, then I completely 
agree with you: This is a very bad idea.  The content of the crown clade will 
change dramatically depending on how close monotremes are to placentals and/or 
marsupials.  And we know how monotremes tend to jump around the tree...


Thus, a whole lot of fossil taxa that have traditionally been considered 
'mammals' may be excluded from Mammalia (crown-group) through no fault of their 
own, simply because the position of monotremes changed relative to placentals 
(or marsupials).  This goes against the grain of 'historical usage', including 
continuity with the paleontological literature.


The same applies to Monotremata, if this clade is limited to the crown clade.  
Time-honored monotremes like _Steropodon_ and _Kollikodon_ would not be 
considered monotremes if Monotremata is restricted to the crown group (platypus 
+ echidna).  


Ditto for Aves.  Defining the clade Aves to be the crown group would exclude 
fossil birds such as _Archaeopteryx_, _Hesperornis_ and _Ichthyornis_, all of 
which have traditionally been included in Aves.


In general, I've never understood the attraction of crown clades.  A crown 
clade is anchored on those species that just happen to survive to the Present.  
There is no phylogenetic principle at work here.  


Well, I'm glad I got that off my chest.

Cheers

Tim