Re: [asa] Transitions

From: David Campbell <>
Date: Fri Dec 08 2006 - 15:13:43 EST

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

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

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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Received on Fri Dec 8 15:14:09 2006

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