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Re: Segregated vs age-mixed sauropod herds
You are right, they permit to infer neontological features to all
extinct members of the group on the basis of parsimony. Other problem
is that the soft-tissue or physiological features that characterize
the crown group, are actually not unambiguous autapomorphies of that
group. Only that you can say "all mammals lactate" (but, although it
is more parsimonious to assume mesonychids and notoungulates lactated,
it is no sure). Not that against crown groups, the stem-groups do not
get rid of the same ambiguous optimization problem.
2010/3/17 T. Michael Keesey <firstname.lastname@example.org>:
> On Tue, Mar 16, 2010 at 10:32 PM, Tim Williams <email@example.com> wrote:
>> 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.
> 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