Dennis Sweitzer wrote:
>Seems to me there can be a lot of odd & quirkly phylums that have survived
>for millions of years in obscurity, and never quite show up in the fossil
>record. Prehaps, in a couple of decades unambiguous evidence of the phylum
>will be found in ancient deposits, and then a whole host of other fossil
>features (say, on trilobite lips) will be understood as being of this
phylum.
This brings up the preservation question in the discussion of the pattern
of evolution of new taxa. Many living phyla are represented by soft bodied
forms (nearly half are "worms") that have very poor preservation potential
and very poor or non-existent fossil records. Thus, when writers declare
that the fossil record shows that all living phyla appeared at the
beginning of the Cambrian they are making an _interpretation_ of the fossil
record. They are assuming the presence of phyla that have no fossil record
in the Cambrian.
The incompleteness of the fossil record can be illustrated by the phylum(?)
Conodonta. This is a very abundant and diverse group of organisms
represented in 300 million years of the fossil record by tiny tooth-like
structures. Because of their worldwide distribution and abundance they are
one of the primary tools for biostratigraphic correlation. Despite their
abundance, until recently, the organisms to which these structures belonged
were completely unknown. Two specimens of the worm-like conodont animal
have now been found, one in Carboniferous rocks and one in Silurian rocks.
Only two specimens are known of a whole phylum(?) of marine organisms known
to be extremely abundant and widespread over a great period of Earth
history. If they did not have mineralized tooth structures we would have
known nothing of their existence!
My point is simply that interpretation needs to be distinguished from data
when discussing evolutionary patterns. The fossil data by itself does not
show the appearance of anywhere near all living phyla in the early
Cambrian. Secondly the problem of retrospective classification I discussed
previously also confuses the issue. If a paleontologist lived in the
Cambrian, different phyla and classes would have been be grouped together
because of their similarity. At least one modern author has proposed the
grouping of most scale-bearing worm-like Cambrian forms into a single
phylum.
Keith
Keith B. Miller
Department of Geology
Kansas State University
Manhattan, KS 66506
kbmill@ksu.ksu.edu
http://www.ksu.edu/~kbmill/