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Re: Segregated vs age-mixed sauropod herds
On Thu, Mar 18, 2010 at 6:57 PM, Tim Williams <email@example.com> wrote:
> By contrast, under Sereno's definitions, _Steropodon_ and _Kollikodon_ would
> be non-monotreme prototherians.
Or, more succinctly, stem-monotremes.
> Similar to his approach to Prototheria/Monotremata, Sereno defines Allotheria
> as the stem-based clade that is more inclusive than the node-based
> Multituberculata (except that unlike Monotremata, is not a crown clade!)
> I take your point, and it's a good one. ÂBut I think it's okay to depreciate
> the value of the term 'mammal'.
Why? I still don't understand what is gained.
> ÂAfter all Mammalia is just one of many clades: Mammaliamorpha,
> Mammaliaformes (anchored in _Sinoconodon_), Theriomorpha, Theriiformes,
> Theria... Âetc. ÂAlthough we cannot be certain when certain mammal traits
> evolved (e.g., determinate growth, lactation, diphyodonty, pinnae), it is
> unlikely that all of these traits are exclusive to crown mammals.
But the full suite of such characters (including genetic characters)
*is* exclusive to the crown clade, or at least to a clade that is for
all practical purposes indistinguishable from the crown clade.
> To me, the danger of crown groups is that they have the (completely
> unintentional) tendency of emphasizing typological differences: in this case,
> mammals vs non-mammals.
Huh! I would have said that these "in between" definitions you are
advocating are far more typology-based. You're establishing an
arbitrary perimeter around a representative group (the crown clade)
and and including anything that is conceptually "close enough" to that
representative group. That sure seems like typological thinking to me.
> To return to dinosaurs, I think there is a similar danger to defining Aves to
> be the crown group. ÂBirds were not cut from whole cloth, and individual
> 'avian' characters first appeared at different stages in evolution. Â
> Attaching the name Aves to the clade that includes _Archaeopteryx_ and modern
> birds (rather than to just the crown group) is certainly arbitrary; but I
> think this is a good thing.
Even if you don't want to use that name for the crown clade,
_Archaeopteryx_ is a lousy specifier.
>ÂIt underlines the fact that most of the traits that make birds so distinctive
>in the modern world (feathers, flight, furcula, pygostyle, etc) were first
>acquired by taxa that are long extinct, and these traits were merely inherited
>by modern birds.
No, it doesn't, because at least one of those traits was acquired
*within* the _Archaeopteryx_ node. (Possibly two.)
You're absolutely right that the traits we associate with living
groups were acquired in a piecemeal fashion. But I think the best way
of emphasizing this is to recognize the relationships of crown clades
to total clades (and thereby to stem groups). Restricting a common,
originally neontological name (e.g., "Aves") to a crown clade allows
us to recognize the clade which exhibits *all* of the relevant traits
(except, of course, where secondarily lost). Having a recognizably
related name (e.g., "Pan-Aves") refer to the corresponding total clade
allows us to recognize the largest clade in which any of these traits
may have originated. And then it becomes a simple matter to recognize
the stem group (e.g., "stem-avians") and know that that's where to
look to pin down the origins of the crown clade's traits.
In this way, I see crown clade names as sort of "fenceposts". They
mark certain ancestors which we have a wealth of information about.
For example, we can infer that the mammalian ancestor lactated and had
pinnae; we cannot infer this for the mammaliaform ancestor, the
theriodont ancestor, the therapsid ancestor, the eupelycosaur
ancestor, etc. We can also infer a lot of things about the amniote
ancestor. And then between these two ancestors, these "fenceposts", we
can start to piece together the history of trait acquisition.
T. Michael Keesey
Technical Consultant and Developer, Internet Technologies