In a recent post, ( Sat, 10 Jun 95 09:34:01 EDT - topic
Re: Gradual Morphological Change) Stephen Jones said in response
to something that Glen said: "Yes. If Glenn's models were true,
we would expect to see continuing "Cambrian Explosions". Why was
there only one?"
Although the Cambrian explosion was a dramatic one and one
that does not fit well with a gradualistic model, it is not true
that there were not other sudden increases in Taxa at various
times in the fossil record. I might also add that most of the
chaps studying the fossils of the Cambrian would say it is less
dramatic than was previously thought.
In any case one of the characteristics of the fossil record
is that organism appear, and usually appear suddenly and well
formed. To quote E. N. K. Clarkson second edition of Invertebrate
Paleontology and Evolution (and he is not a YEC, but author of a
standard text in Invertebrate Paleo that I use when I teach it
here at Dordt), "New types of structure, and new grades and
systems of animal organization, appear very suddenly in the
fossil record...... Even in the first representatives of these
groups, all the systems of the body, and all the anatomical,
mechanical, physiological and biochemical elements therein, have
to be precisely and harmoniously c- coordinated from the very
beginning. The origination of higher taxa in this sense, remains
the least understood of palaeontological phenomena."
Sometimes loads of new taxa will suddenly appear in the
strata. In some case this is compounded by the fact that the new
layer may represent a different paleoenvironmnet than the last
and that would result in sampling a previously unsampled strata -
but often they are not found elsewhere. Let me mention a few.
Angiosperms in the Cretaceous show a virtual explosion. After a
few earlier fossils you suddenly quite loads and loads of kinds.
The same thing can be said for birds. If you really want to see
the dramaticness of this look at a publication the GSA put out
that shows the ranges of all the genera. This book should be in
any major University Library. These are only a few. Many of the
fossil groups tend to almost explosively appear with a lot of
variety.
This does still not mean that the Cambrian with its quite
abrupt appearance of hardshelled organisms is not unique in the
largeness of the scale. I just wanted to correct the impression
that Steve's post may have left that this was the only dramatic
appearance in the fossil record.
-- :James F. Mahaffy e-mail: mahaffy@dordt.eduBiology Department phone: 712 722-6279Dordt College FAX 712 722-1198Sioux Center, Iowa 51250