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RE: The Diversity of Life (was LIONS & TIGERS REVISITED



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> dbensen
>
> I _had_ wondered about that.  In The Diversity of Life, Edward O.
> Wilson makes
> much of the supposition that life is more diverse now then ever.
> He even has a
> nice graph that shows the number of species increasing though
> time.  He must have
> based his theory on _something_, but what?

Do you mean the graph on p. 191?  If so, this is the (extremely significant)
graph of abundance of marine "animal" families (also includes protozoans)
over time, compiled by the late, great Jack Sepkoski.  Although there are
some criticisms of the particulars of this plot (it does, after all, use
"families" as if they were equivalent units, rather than species; not all
the taxa in question are monophyletic; etc.), the large scale structure does
seem to hold.  There ARE more observed fossil groups in the Late Cenozoic
than in the Cambrian, for instance.

(In fact, this plot is one of the most famous in general paleontology: check
out almost ANY modern general paleontology or historical geology textbook
for more details on it, and in particular about the Three Evolutionary
Faunas, an important derivative of the analysis from which this curve is
drawn).

Does this mean that there was indeed less diversity in the past?  Obviously
there are many mitigating factors:
*Non-preservation of many groups of animals, particularly those with
soft-parts
*Mis-identification of species from extinct clades, relative to those with
modern representatives allowing a better judgement of within-species level
variation
*Non-inclusion of ghost lineages in the data base
*Non-discovery of fossil localities, and more generally the fact that we
have much better exposure of Late Cenozoic units than Middle Jurassic units,
or of Middle Jurassic units than Early Cambrian units, and so forth: an
aspect of a problem called the Pull of the Recent
*The fact that the database used to generate these results was formed in the
late 1970s and early 1980s: there has been a lot of work in the field since
then.
*and others.

Okay, would these corrections change the plot?  Can't say without doing it,
and while I can think of some ways of dealing with these problems, others
would be extremely problematic.

At present, though, I think it is fair to say that the obervation is that
diversity of marine organisms has changed over time, and that the observed
level for the Late Cenozoic (exclusive of modern faunas, so soft-bodied
critters don't get involved with the problem) is higher than at any earlier
point in the record.

>  I hear the fossil
> record for plankton
> is pretty good (diatoms and all that).  Has _their_ diversity
> been increasing?

Well, "plankton" is an ecological category (floaters incapable of swimming)
rather than a taxonomic category, but yes: many groups of single-celled
organisms have an excellent fossil record and do show various peaks and
declines.  For some groups (coccolithophorids) current diversity is not very
high compared to the glory days of the Cretaceous; for others (globigerinid
foraminferans, diatoms, etc.) current diversity as is high as ever.
(However, there is an indiciation that diatom diversity may have been higher
in the past, but their tests (shells) become altered in preservation into
silica silt grains).

Incidentally, I recommend folks check out the systematics of the eukaryotes
page of the Univ. California Museum of Paleontology website
(http://www.ucmp.berkeley.edu/alllife/eukaryotasy.html) if you want some of
the current thinking of the position of the various groups.  Futhermore,
each of the individual pages gives details on their fossil record, anatomy,
etc.

Finally, there are other large-scale secular changes in diversity: beetles
make up an estimated half of all currently known species (note that beetles,
as land animals, are not included in Sepkoski's database and are not
responsible for the increase in diversity), but Coleoptera only goes back to
the Permian and their great increase in species number did not occur until
much later.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796