Re: Segregated vs age-mixed sauropod herds

["T. Michael Keesey" <keesey@gmail.com>]

On Thu, Mar 18, 2010 at 6:57 PM, Tim Williams <tijawi@yahoo.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!)

Unfortunately...

> 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
Glendale, California
http://tmkeesey.net/