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Species, ancestors, etc. (was Re: Stratigraphy, biogeography & cladograms)



At 02:06 PM 1/20/99 -0600, Justin "Thescelosaurus" Tweet wrote:

>>Not only that, but the ancestors are paraphyletic taxa, since they 
>>don't
>>include any of their descendant species (let alone all of them).
>
>   That reminds me of an itchy question I've had for a while: let's say
>that *Pentaceratops sternbergii* is a direct descendant of *Chasmosaurus
>mariscalensis*.  I'm not saying that it is factually, I'm just using it
>for a scenario.

Fair enough.

>   Now, this would mean that *Chasmosaurus* is a paraphyletic genus if
>*Pentaceratops* is not included, right?

Yes, if by _Chasmosaurus_ you mean the whole assemblage containing _C.
mariscalensis_, _C. belli_, etc.

>And, then wouldn't *C.
>mariscalensis* be a paraphyletic species if it didn't include the newly
>renamed *C. sternbergii*

Yes.  True ancestors are by their very nature paraphyletic.

>(and who gets to keep their species name? *C.
>mariscalensis*, because it's been in *Chasmosaurus* longer and would be
>paraphyletic without *C. sternbergii*, who doesn't need *C.
>mariscalensis* to be complete, or *C. sternbergii*, because it is an
>older species name?)? 

If you regard these as different species (one ancestral to the other), then
they both keep their names.  If, on the other hand, you feel for some reason
that you should unite these all under the same species name, then the oldest
stays (i.e., _sternbergii_).

There are several possibilities for the taxonomy here, even if we agree on
the tree topology _C. belli_ + (_C. marisc._ + _P. sternbergii_). (For
purposes of this hypothetical situation, _C. russelli_ and other
_Chasmosaurus_ species are assumed not to exist...  Sorry, guys.).

I) Reserve _Chasmosaurus_ for _C. belli_ and expand _Pentaceratops_ to
include the new combination _P. mariscalensis_.

II) Include all three under the same genus name.  By priority, that name
would be _Chasmosaurus_.

III) Create a new generic name for _mariscalensis_ ("Eopenta...".  Oh, wait,
I complain when other people propose new taxon names on the list, so I
should follow suit.  Sorry.)

IV) Leave the names the way they are, and accept a paraphyletic genus
_Chasmosaurus_.  (This has the disadvantage of loss of information (i.e.,
that _C. mariscalensis_ is closer to _Pentaceratops_ than to _C. belli_),
but reference to a cladogram would explain the relationships).

These four possibilities are all valid, and your choice has more to do with
personal taste than anything else.  You will find examples like all three of
these in the current literature.  It is the evolutionary relationship which
is the more imporant thing.

>   *C. mariscalensis* has to have had an ancestor, too.  Wouldn't one
>have to assume the name of the other to make the ancestor monophyletic?  

No: ancestors BY THEIR NATURE have to be paraphyletic.  

>   What the thing I'm getting at is, shouldn't every living thing on
>Earth have the same generic and specific name as the first living thing
>on Earth (which I don't think will ever be found), in order to make that
>first living thing monophyletic,

Would kind of defeat the purpose of generic and specific names...

>or can species get away with things that
>families can't?

Yep, under certain species concepts.  (Not to say that you are re-inventing
the wheel, but just about EVERYONE goes through this at some point.  There
is a vast literature on species concepts, species taxonomy, etc.).

One way of thinking about it is as I alluded to (and George came out and
said) earlier today: species are cross-sections through lineages.  Lineages
are the true historical entities; species boundaries are arbitrarily (though
hopefully wisely!) chosen segments of those lineages, just as named clades
are arbitrarily (although hopefully wisely) chosen branches of the same.

>   I've heard the approach where specific and generic names are abandoned
>and the specimen numbers (like USM 1180, or whatever) used instead kicked
>around before, and I hope that doesn't happen, if only for my memory. 
>It's a lot easier to me to remember *Triceratops horridus* than all the
>catalog numbers assigned to animals once designated *T. horridus*. 
>However, I don't know if that's correct.-*Thescelosaurus* 

If you are referring to what Mike Keesey was talking about, he wasn't saying
we should abandon the name _Triceratops horridus_.  Instead he was
suggesting (as de Queiroz has before him) that we actually get down to the
nitty-gritty and (under phylogenetic taxonomy) define clade names using
relationships between specimens rather than species or higher level of the
taxonomy.

(However, if you do go further in studying dinosaurs or any taxon, you will
probably find yourself memorizing specimen numbers whether you like it or
not!! :-).

Hope that helps.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661