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In a recent message, Barry Roth commented: "Might I venture to suggest that the
phylogeny of phytosaurs [or that of any other group of organisms of
biostratigraphic interest] should be more than a side issue when considering a
zonation based on them?" This is obviously true, but misses the rationale for
work in biostratigraphy.
Biostratigraphic zonation depends on the identification of reliable datums in
the area of application. Datums (horizons of first appearance or last
appearance) are a product of evolution (species origin/extinction) and/or of
change in geographic range (migration into or out of an area). Because of this
multiple control, two critical steps are necessary to generate good
biostratigraphic zonations: 1) identification of datums and 2) judging the
suitability of specific datums for zonation. This time-consuming work is
fundamental to success.
When a lineage is evolving rapidly in the same region, high resolution zonation
can correspond to phylogeny determination. However, much evolution does not meet
this dual requirement (rate and location). Allopatric speciation (and geographic
out-migration) is common, making for abrupt appearances in a basin or province.
Ancestor and descendant species and the transitional forms cannot be relied upon
to be present in the area of study. (Pelagic microorganisms most often come
close.) In the normal set of circumstances, a biostratigrapher should focus
his/her attention on datum evaluation and hope that phylogeny is helpful to
establish zones - not expect it to. Phylogeny serves as a background check,
indicating where second stage study could be directed.
Biostratigraphy is a fundamental area of paleontology - it provides us with the
tools for age determination which make other pursuits possible. Much
biostratigraphic work remains to be done, so let's emphasize the importance of
sound procedures in biostratigraphy and not subsume the practice into other
aspects of the science. Developing good biostratigraphic zones is highly skilled
work, with value equal to, or greater than, that of developing good phylogenies.
_________________________________________________________________________
Thomas E. Yancey
Dept. Geology & Geophysics
Texas A&M University, College Station, TX 77843-3115
Voice: 409 845 0643 Fax: 409 845 6162
email: tyancey@tamu.edu
--------------------- (misquoted from)
Complex problems have simple, straightforward, wrong solutions. (H.L. Mencken)
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