Re: index fossils
David Campbell (bivalve@mailserv0.isis.unc.edu)
Sat, 19 Jun 1999 10:52:54 -0400
>>I think I asked this before but can't remember how the responses went.
>>Why couldn't high stress conditions have caused morphological changes in
>>the species, which would then propogate around the oceans? Why couldn't
>>these changes have occurred in a relatively short period of time?
>
>It is the settling rate of these nannofossils that prevents their rapid
>burial. Remember Stokes law for something like a nannofossil requires that
>it would take up to 51 years for it to fall to the ocean floor. The flood
>only lasted one year. And there is total separation of morphological form
>which would not be the case for stress induced morphological change.
>Believe what you want, Reality only allows certain things to be true and
>requires other things to be false.
Settling can be speeded up a little if the nannofossil falls to the bottom
as part of a fecal pellet or other, larger object. On the other hand, it
takes a little while (currently, centuries) for something to be distributed
all around the globe. There are hundreds of global changes in various
kinds of index fossils throughout the geologic column. Each one must be
distributed globally and eliminated globally in order. Similar patterns of
replacement of one species by another occur in terrestrial and marine
species ranging in size from a few microns to tens of meters.
>Also,
>>do we see a gradual transition between species, or are only the two
>>distinct types found?
Some of both.
David C.