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Re: Segregated vs age-mixed sauropod herds



On Tue, Mar 16, 2010 at 10:32 PM, Tim Williams <tijawi@yahoo.com> wrote:
>
> In general, I've never understood the attraction of crown clades. ÂA crown 
> clade is anchored on those species that just happen to survive to the 
> Present. ÂThere is no phylogenetic principle at work here.

Popular old names like "Mammalia" originated in the neontological
literature and are still more prevalent in use there. Inferences are
commonly made based on living forms, and these can go awry when
members of the stem group are referred to by names commonly used to
refer to the crown. For example, we all know that mammals have
determinate growth and are diphyodont -- oh, but Sinoconodon had
indeterminate growth and was polyphodont. Did it lactate? Was it
endothermic? Did it have pinnae? Who knows? What does it buy us to
anchor such a common name to an arbitrarily selected member of the
stem group (especially when that leaves us no apparent name for the
crown)? It lessens the number of things we can confidently state about
mammals, and thus depreciates the value of the term.

(Actually, as I've noted before, "Mammalia" is often used in
neontological contexts to refer to *Theria*, but that's probably going
too far ... probably....)

Nomenclature is about communication, and communication is about
information. We have more information about a crown clade than we do
about more inclusive clades within the corresponding total clade.
Indeed, vertebrate paleontology is, to my knowledge, one of the few
areas where this debate exists at all, since most taxa have much more
poorly-known stem groups (when they are known at all). We're trying to
build a common language across disciplines, and crown clades are the
only ones that we know about at all in many disciplines.

-- 
T. Michael Keesey
Technical Consultant and Developer, Internet Technologies
Glendale, California
http://tmkeesey.net/