[asa] Re: Fossil record was Transitions

From: James Mahaffy <mahaffy@dordt.edu>
Date: Sat Dec 09 2006 - 11:27:49 EST

I'm aware that there are single-celled creatures that produce tests that
can be traced--if they are distinctive enough. I've heard of the White
Cliffs of Dover and immense deposits of diatomaceous earth. But are
there
any fossils of Entamoeba, Volvox, Stentor, Giardia, Paramecium, Euglena,
Myxomycetes, etc., which don't secrete tests, that allow tracing their
evolution?

Softbodied single celled are not abundant in the fossil record, unless
they were algae or blue-greens (cyanobacteria) preserved in
stromatolites. Some of the earliest preCambrian fossils are in these
two groups.

Parasites like Entamoeba and Giardia are concentrated in the intestines
of their hosts. I don't know their record, but it could be that we
haven't looked that much. You would have to find good coprolites that
still had lots of organic material. Might well be some around, but I am
not sure they have been looked for (you would waste years looking for
things you might not find - but if you did it would be a great PhD
these). David Dilcher did his PhD thesis on the fungi growing on early
angiosperm leaves.

Actually Volvox has a zygote that should preserve fairly well, but then
it is a not that common a freshwater alga and the zygotes may even be
missed in fresh water. Now if you want to draw a nice tree in the
Volvocine algae [which I do not] you would probably use living
flagellates in the group. You have single celled ones, ones with about
4 in a colony and others up to the thousand or so in Volvox.

The fungal groups have pretty good records. They have identifiable
spores and many living on decaying organisms can leave behind some
vegetative some are distinctive. structures. Mostly it is hard to
identify from vegetative structures but sometimes you get identifiable
structures on rotting material (good preservation potential)

Among somewhat larger creatures, are there fossils of
jellyfish,

Jellyfish have a long but sparce record. Fortunately the fossil record
has localities that occasionally preserve a lot of softbodied chaps.
One of these windows is in the preCambrian Edicarian flora which in the
traditional interpretation (according to Clarkson) has 15 species of
jellyfish (medusa). However, you are right, they generally are not
preserved (just a few early ones). Another hand chitinous hydrozoan are
more easily preserved and have a fossil record back into the Ordovician.
 And of course something like corals have a great record and I suspect
we are pretty sure we know what we had (different types of corals -
tabulates and horn corals in the Paleozoic - modern corals found first
in the Mesozoic). It is pretty hard not to preserve a coral unless it
dissolves.

roundworms and flatworms?

Nematodes should be there more often then they are recorded. they can
be found in amber and apparently someone found some in the Paleozoic.
See:
http://www.nola.com/living/t-p/index.ssf?/base/living-7/116435034131380.xml&coll=1&thispage=2

Sounds like hardly anyone studies them.

I don't think there is much of a record of flatworms (which is to be
expected) but see:
http://www.sciencedaily.com/releases/1999/03/990322062150.htm

I don't remember any of them with
calcareous or siliceous parts. I know that there are cephalopods with
shells, which is why I specified octopods. I now understand that there
are fossils of shelled (externally or internally) molluscs from which
they evolved. Any unshelled samples?

Actually in the group that includes the octopus and squid it is only the
belemites that had a spearlike internal structure that preserve well and
give us a good record. I am not sure why the chitinous beak is not
preserved a bit more often but even if they were they would be VERY rare
compared to brachiopods etc and perhaps overlooked.

[snip] There is clearly the
element of "luck" in encountering fossils.

Yes but I have tried to indicate that actually for many groups we have a
very good record that is NOT that spotty. You can usually predict which
ones will not likely to be preserved. It is just not that random. And
now I had better get back to grading, but this has been fun.

James Mahaffy (mahaffy@dordt.edu) Phone: 712 722-6279
498 4th Ave NE
Biology Department FAX : 712
722-1198
Dordt College, Sioux Center IA 51250-1697

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Received on Sat Dec 9 11:28:27 2006

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