Keith thought he had posted this to ASA list but sent it only to me by
mistake and asked me to post it since I still had a copy.
JFM
Just a few comments about James post.
I have not read Glens web article, but I have followed quite closely
the literature on the Precambrian/Cambrian transition. Many of the
critical discoveries relevant to this issues have been descirbed only
within the last two years - too recent for even Clarkson's text.
I will briefly describe some of these below:
1) A fossil called Kimberella is found in the Late Precambrian
ediacaran and is interpreted by many workers as being a very primitive
mollusk.
In fact it appears quite close to the hypothetical ancestral mollusk
previously suggested (see Fedonkin & Waggoner, 1997, The Late
Precambrian fossil Kimberella is a mollusc-like bilaterian organism:
Nature, vol. 388). Scratch marks previous thought to possibly be
arthropod appendage marks are now believed to represent radula marks
from these organisms.
With other known body fossils, the Late Precambrian record of living
metazoan phyla now includes sponges, coelenterates, and mollusks. With
less confidence some body fossils are attributed to echinoderms.
Several bilaterian ("worm") phyla are present although there is dispute
as to the their taxonomic identity. There are also a host of
beautifully preserved metaphytic algae. This leaves brachiopods,
arthropods, annellids, and chordates as the living skeletonized
invertebrate phyla that currently are known to first appear in the
Cambrian. Bryozoa are first known in the Ordovician. There does appear
to be a close anatomical similarity between annellids and the primitive
mollusks of the Cambrian.
2) There are also new discoveries that are pushing the appearance of
metazoans back before the ediacaran. The most spectacular of these is
the discovery of phosphatized embryos already discussed on this forum.
3) The transition from univalved helionellid mollusks
(monoplacophorans) to bivalves through a group of rostroconchs in the
early Cambrian seems now quite well established. There are now know
intermediate specimens that occur in the chronological position.
(Gubanov, Kouchinsky, and Peel, 1999, The first evolutionary-adaptive
lineage within fossil molluscs: Lethaia, 32:155-157.)
3) Recent work does strongly suggest the phylum-level transition from
lobopods to arthropods. The fossil sequence is from the onychophora,
to armoured lobopods such as Microdictyon, to gill-bearing lobopods such
as Kerygmachela, to the stem-group arthropod Pambdelurion, to
anomolacarids and euarthropods. What is really exciting about this
proposed transition is that the skeletal/muscular systems are known in
several of these taxa - onycophorans, Kerygmachela, Pambdelurion, and
euarthropods. What this sequence illustrates is a change from
peripheral muscle with an incompressible haemocoel, to a combination of
peripheral and skeletal muscle, to a loss of peripheral muscle and the
mineralization of the cuticle. (Budd, G.E., 1998, Arthropod body-plan
evolution in the Cambrian with an example from anomalocarid muscle:
Lethaia, 31: 197-210.)
Keith
Keith B. Miller
Department of Geology
Kansas State University
Manhattan, KS 66506
kbmill@ksu.ksu.edu
http://www-personal.ksu.edu/~kbmill/
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