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Re: More tyrant Q & A's



At 11:27 AM 12/9/98 -0500, Thomas R. Holtz, Jr. wrote:
>>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.

Well, for congeners, something close to that is, in my opinion, necessary.
Sufficient is harder, as sexual dimorphism can equal or exceed
interspecific variation (e.g. house sparrows in parts of the US).

Strictly speaking, I do not remember the exact numbers, but there was an
interesting study of sympatric parrots some years ago that showed what
appears to be the basic pattern for closely related species. Unfortunately
my library is in such disarray I cannot find the citation to determine the
actual ratio observed.  I seem to remember it actually being a bit *more*
than a 20% difference in length - I really wish I could find the material.
(I also seem to remember they actually measure body weight, not length, or
maybe they measured both ...)

>  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).

I agree, it is not always clear cut.  Especially when sample sizes are small.

There are some cases of subtle forms of niche partitioning even in
mid-sized forms, like auks and auklets.  In trying to find the citation on
the parrots, I ran across a bit on how auks partition feeding space by
feeding at different distances from the nesting grounds.  But these birds
also differ in shape and size of the bill, so there are additional differentia.
>
>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.

I do see how this might happen, especially, as you say, if only a few were
known.  However, they generally inhabited different specific habitats in
the each area, with lions in more open communities, and tigers in more
forested places.  This would, I think, tend to put them in different
*facies* at least, within any given formation.  Still, this *is* a subtle
cue, and my be hard to establish; though I suppose it may be one additional
factor supporting distinguishing the Ghost Ranch theropod from the
Petrified Forest one.
>
>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.

I am certainly still treating them as such in my Web site :-)

>>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.

Yeah, as always, interpreting the fossil record is a tightrope.  There is
lots of information there, but there is lots of noise, and lots of missing
data as well.  Finding the pattern through the murk is often difficult.
But then, I always did enjoy a challenge. (If it were *easy* it would be
boring :-)

[Ecological factors are one reason I suspect the old species level taxonomy
of ceratopsians is massively oversplit].

--------------
May the peace of God be with you.         sarima@ix.netcom.com