Re: [asa] Re: Fossil record was Transitions

From: David Campbell <pleuronaia@gmail.com>
Date: Sat Dec 09 2006 - 13:55:42 EST

> > I'm aware that there are single-celled creatures that produce tests
> > that
> > can be traced--if they are distinctive enough....But are
> > there any fossils of Entamoeba, Volvox, Stentor, Giardia, Paramecium,
> > Euglena, Myxomycetes, etc., which don't secrete tests, that allow tracing
> > their evolution?

If there is no durable test of some sort, then the fossil record is
pretty patchy. A few such things are known from amber or inside
Pleistocene frozen animals or as impressions where another organism
grew on top of it or the like. Some higher groups are chemically
distinctive enough to produce diagnostic markers detectable in the
geochemical record.

> > 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).

Coprolites actually would be a very good place to look, if you don't
mind looking at them. The high phosphate levels are excellent for
rapid preservation, so traces of soft features in the remaining food
bits are not uncommon (muscle fibers, etc.).

> > 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?

I know there exists a fossil octopus with just the organic remnant of
a shell-no calcareous part. Octopi also make distinctive holes in
shells when they prey on them, and traces of such predation can be
found in the fossil record.

> > 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.

There are a number of things that aren't true belemnites that also
have the internal shell. Modern cuttlefish and Spirula still have
calcareous internal shells among the squid-like organisms; they have a
fairly continuous fossil record.

There's a reasonable fossil record of cephalopod beaks, sometimes as
fish stomach contents.

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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Received on Sat Dec 9 13:56:28 2006

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