At 15:06 10/8/02 -0300, Rod Savidge wrote:
Matthew,
I certainly was not suggesting that the fossils at Mistaken Point might
be vascular plants! I was merely using a highly complex
non-photosynthetic flowering vascular plant as an example of how
versatile nature can be. Sometimes it surprises us. In
biological classification today (which still tends to be arbitrary and
recently has undergone immense change due to increasing insight into DNA
affinities), I doubt very much that 'algae' not able to photosynthesize,
whether single or multi-celled, would be considered as algae.
Indeed, even some photosynthetic algae are considered to be more closely
related to protozoa than they are to other algae. Water molds,
slime molds and chytrids are plant-like organisms typically lacking of
photosynthetic pigments.
.........snip....................
I still think it is unlikely that the entire biota represents some
non-animal group, whether fungi or protozoans. Their morphology is
more complex than is typical for these groups (although I am sure they
could theoretically fit within the known range of complexity), with the
possible exception of
Ivesia (the pizza disc/bubble mat
form). My main objection is that their community ecology (species
richness, abundance, diversity, tiering, spatial patterns, and spatial
interactions) is just so similar to that observed from other marine
animal communities. And I can just simply restate my previous
objection: I don't see why it is necessary to speculate that the Mistaken
Point biota represents the one and only example from the fossil record of
a community dominated by water molds or slime molds, or anything other
than animals. Older fossilized embryos from China imply there were
animals present so it seems unnecessary to discard the animal
hypothesis. Of course it is important to keep an open mind about
alternative possibilities, but any argument that seeks to classify the
entire biota as a non-animal group will need special proof, I
think.
The preferred orientation of the
fossils could equally be a result of the direction of current flow over
the bedding surface induced in response to the volcanic eruption.
The oriented fossils are all essentially fusiform in shape and, I think,
their long axes would tend to become aligned parallel to the flow
direction even if they were settling from above. I did note one
frond-like fossil with an orientation at least 20 degrees different from
its neighbours.
There are also rare
Charniodiscus and other fronds with
orientations 180 degrees from the typical frondose mean on any given
surface, but that doesn't take away from the overwhelmingly consistent
felling direction. I can't say much on the implications of the
orientation data since the paper is in review now, but in most cases they
don't appear to relate to the volcanic ash turbidites.
............snip...........
I disagree here. The frond-like organisms with putative stalk and
holdfast would tend to drag, the bulbous holdfast serving as an
anchor. The spindles, tapering from midpoint to both ends, would
tend to tumble and become variously oriented, as you note they are.
If spindles tumbled in the current they should adopt the most stable
orientation - perpendicular to flow - or parallel to flow if they were
deposited from suspension. But neither of these modes are
observed. There is also no evidence for dragging of frondose
species - in some cases spindles are even observed to grow overtop frond
holdfasts.
Regarding morphology, I
"heard" descriptions of bilateral symmetry on the field trip,
but I looked carefully and rarely saw any really convincing evidence for
that. Instead, I saw abundant evidence for non bilateral
development (I have photos!). I also saw what I believe are
different stages in development of a single species, although as I
understand it they are being interpreted as different species.
Whether plant or animal, it is normal as an organism grows for its
shape/appearance to change, sometimes profoundly. The
morphology of Mistaken Point taxa may be rigidly developed, but the
actual specimens show considerable variation. I think this
question needs a lot of very careful study.
Although the organisms were certainly not bilaterians, I think that many
of them display some form of bilateral symmetry (
Charniodiscus,
spindles) once preservational effects have been removed. I would
also disagree that there is considerable variation within species - most,
once retrodeformed, have remarkably consistent shapes and morphological
features. And with the exception of the "bubble mats" and
"pizza discs," which are now interpreted as preservational
variants of
Ivesia, I never observed intergradation of forms or
anything which could be interpreted as ontogenetic changes.
I really appreciate your
response.
Rod Savidge
Matthew E Clapham
Matthew E. Clapham
Department of Earth Sciences
3651 Trousdale Pkwy
University of Southern California
90089-0740
Phone: (213) 821-6291
Fax: (213) 740-8801
clapham@usc.edu