Re: [asa] Transitions

From: D. F. Siemens, Jr. <dfsiemensjr@juno.com>
Date: Fri Dec 08 2006 - 16:32:43 EST

On Fri, 8 Dec 2006 14:13:43 -0600 "David Campbell" <pleuronaia@gmail.com>
writes:
> There are certain amoeba-like or flagellate groups that do have a
> good
> fossil record, though there are also some that have such generic
> hard
> parts (mineralized or tough organic) that it's impossible to
> identify
> them. Most studies on the microfossils focus on distinguishing
> species for stratigraphic purposes, so relatively little has been
> done
> on the evolutionary framework. As an extreme example, some of the
> calcareous nannofossils that survived the K/T are put in different
> families before or after because they are studied separately.
> Nevertheless, there are some good transitions known. I
> specifically
> recall studies on the planktonic forams just after the K/T.
> Perhaps
> only a couple species survived. Very shortly afterwards, one
> lineage
> starts showing significant variability leading into several
> different
> lineages of Tertiary forams.
>
> The invertebrate groups with good fossil records have plenty of
> good
> transitional forms. In the Cambrian, the armored lobopods,
> anomalocariids, etc. are intermediate between the phyla Onychophora
> and Arthropoda. Mollusks generally have good fossil records (slugs
> being one obvious exception) and have good transitions between most
> major groups. The transition between cap-shaped "monoplacophorans"
> s.l. and bivalves and between the cap-shpaed shells and cephalopods
> are particularly well documented. For the bivalves, there are
> narrow,
> slightly coiled cap shaped shells and then progressively narrower
> and
> less coiled forms. Finally, there is one with distinctly thin,
> flexible shell along the dorsal edge. Make that thin, flexible
> shell
> into flexible organic material and you have a bivalve, and the
> oldest
> bivalves look much like them.
>
> For the cephalopods, increasingly tall conical shells eventually
> led
> to one with chambers partitioning off the back end. The only step
> remaining to becomming a cephalopod was to develop a tissue
> connection
> to the chambers so that the animal could control what was in them.
>
> Both of these fit together tightly stratgraphically as well as
> morphologically.
>
> There are also good transitions within these. In the lineage
> leading
> to squids, progressive encasing of the shell within the body and
> reduction of the shell can be traced from relatively ordinary
> straight-shelled cephalopods through the belemnites, with just the
> tip
> of the reduced shell emergent through cuttlefish-like forms with a
> well-developed but entirely internal (and much reduced from the old
> external) shell to standard modern squids with just a stiff organic
> remnant. Octopus developed similarly in parallel but internalized
> the shell more rapidly and have now long lost even the organic
> remnant.
>
> In the bivalves, one I've been involved with is the transition
> between
> pearl oysters and oysters. Within the past decade, some of the
> earliest fossil oysters have been found to still have some pearly
> shell structure, unlike the later ones that entirely lost it. DNA
> also clearly allies the oysters and pearl oysters.
>
>
> To the extent that people have looked, there are plenty of good
> transitions in the invertebrates. People care much less about
> them,
> so there's much less attention given.
>
> --
> Dr. David Campbell
> 425 Scientific Collections
> University of Alabama
> "I think of my happy condition, surrounded by acres of clams"
>
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>
>
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? Among somewhat larger creatures, are there fossils of
jellyfish, roundworms and flatworms? I don't remember any of them with
calcareous or silicious 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?

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.

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.
Dave

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Received on Fri Dec 8 17:06:13 2006

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