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Re: More tyrant Q & A's



At 06:46 AM 12/9/98 -0800, Stan Friesen  wrote:

>On ecological grounds, forms that co-occur in one time-place can really
>only be distinct species if they differ sufficiently to be able to minimize
>competition.

Quite.  The problem here is that it is unclear at present how different two
forms have to be to qualify as "sufficiently different".  It would be great
if, for example:

>Given that Sue and Stan come from the same bed cannot be
>different species, unless they are about 20% different in body length.

Were true.  However, is there experimental proof suggesting that 20% is
necessary and sufficient?  That would be great.  Unfortunately, there are
factors such as differences in soft tissues, differences in mating cycles,
etc., that can allow morphological similar taxa to occupy the same space at
the same time (generally with smaller forms, like insects and rodents, mind
you).

For example, tiger and lion skeletons are EXCEEDINGLY similar.  If they were
only known from two dozen incomplete skeletons, I think most paleontologists
would call them the same species.  And, since tigers and lions have shared
the same geographic regions in  historic time (range overlap in western Asia
and India), they could concievably wind up in the same formation.

Not that I am arguing that Stan and Sue are different taxa: I think the case
that they are two individuals of the same species (_T. rex_) is very strong.

>(And that really only applies if T. had deterministic growth - I suspect
>that indeterminate growth ends up forcing even greater differences in
>sympatric congeners).

I, and others (Mike Brett-Surman, for example) might strongly agree with you
here.  (Interesting aspects with regards to hatchling-to-adult size changes,
too...).

>>NOT a remote example, and very germane to the subject.  Some modern species
>>DO vary widely in terms of their morphology.  This is a problem in taxonomy
>>(modern and paleo-): where do we draw species boundaries.
>
>Quite.  And living species that are most similar to one another live in
>distinct geographic areas.  This helps, if one uses this fact in analyzing
>the fossil record.

See above with regards to tigers and lions, for an (admittedly annoying)
counter example.

Something to keep in mind, at least.

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