Re: [asa] Transitions

From: David Campbell <pleuronaia@gmail.com>
Date: Wed Dec 13 2006 - 11:57:18 EST

> But are there any fossils of Entamoeba, Volvox, Stentor, Giardia,
> Paramecium, Euglena, Myxomycetes, etc., which don't secrete tests, that
> allow tracing their evolution?

There are occasional representatives of several different protist
groups that produce durable tests at some stage of the life cycle,
thus providing a small window into the evolution of the group.

> Among somewhat larger creatures, are there fossils of
> jellyfish, roundworms and flatworms? I don't remember any of them with
> calcareous or silicious parts.

The conularids are thought to have been jellyfish-like forms with
phosphatic skeletons. (We know they had phosphatic skeletons; we're
not absolutely sure what they were, but it seems highly likely they
were in cnidarians with a jellyfish-like stage in the life cycle).
Apart from that, there are no extensive hard skeletons in any of those
three, though at least the flatworms may occasionally have spicules.
There are a few fossils in settings of exceptional preservation for
jellies and nematodes; I don't know offhand of any flatworm fossils
but they could turn up in amber, parasites on frozen or mummified
organisms, etc.

> Regarding the Cambrian and Precambrian fossils, were their exoskeletons
> purely organic? I suspect they were fossilized in part because of their
> calcium content. But conditions had to be right for their burial.

Mineralized skeletons first appear in the latest Precambrian and
rapidly became common in the Cambrian. However, tough organic
skeletons can fossilize well-pollen, wood, spores, arthropods, etc.
Incidentally, although calcium carbonate and calcium phosphate are the
most popular minerals, non-calcium mineralized skeletons are around,
especially silica.

> I recall recently reading about some fossilized embryos from China. My
> recollection is that they are unique and that there was no identification
> of what they might have developed into. It doesn't seem that they
> contribute to placement in an evolutionary series. There is clearly the
> element of "luck" in encountering fossils.

They do have developmental patterns characteristic of bilaterian
animals, so there's some evidence of their evolutionary placement.
One locality yielded some further developmental stages suggesting a
worm-like adult. However, there are worm-like animals in most
bilaterian phyla, so that doesn't help much in identifying the
affinities.

-- 
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 Wed Dec 13 11:57:46 2006

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