Re: paleonet non-green plants and Mistaken Point

[Matthew E Clapham <clapham@xxxxxxx>]

Dr. Savage,

I am glad you enjoyed the trip to the Avalon.  Mistaken Point certainly is a fantastic site, both for the exceptional preservation and the large number of in situ census communities preserved (including the oldest known complex communities).  You have raised a number of key questions regarding the affinities of the Mistaken Point biota and their preservation and life habits at Mistaken Point.  These are questions that the Queen's University group has been wrestling with over the years - and there will be several papers coming out over the next year or so which will address many of these points, in addition to what I have said below. 

At 11:04 10/8/02 -0300, Rod Savidge wrote:

Dear Dr. Guy Narbonne,

I want to share with the Paleonet group some of my observations and reflections made on the field trip to Mistaken Point Ecological Reserve this last weekend,  perhaps to enhance discussion at the upcoming Denver conference (which I unfortunately cannot attend).   Firstly, let me say thank you for conducting a most enjoyable field trip.  I truly admired the energy and obvious motivation you and the other organizers expended to share your dedicated research efforts and knowledge with us.    I don't know if there will be future trips to Mistaken Point, but for anyone interested in the origins of eukaryotic morphogenesis, I can certainly recommend a visit there as very worthwhile.  The Avalon in Newfoundland is a fantastic place, especially well appreciated when the weather cooperates!

As an amateur, one of the things that puzzles me about paleobiology in general is the preoccupation with animals at and just prior to the so-called Cambrian explosion, despite the general acknowledgement that algae had appeared on earth about a billion years earlier.  There seems to be an attitude that complex animals had the wherewithal to evolve whereas plants did not.  It doesn't make a whole lot of sense (at least to me), and I therefore participated in this field trip with the idea that maybe some of the Mistaken Point 'animals" in fact were plants.   As I now understand, the tendency has been to see the Mistaken Point biota as early complex animals primarily on the assumption that the lifeforms  were positioned too deep to be able to photosynthesize.   Recall that we discussed non-green plants in relation to the fossils there!  An example of Monotropa uniflora can be found at http://www.rook.org/earl/bwca/nature/herbs/monotropa.htmla (I have my own photos if you would like to show one at a conference).  It is a common flowering (tracheophyte) species found in forests throughout North America.  There are various other species, all non-photosynthesizers, within that genus, and there are a great many other non-green species of vascular plant that are very well documented.  Every one that has been investigated biochemically has been found to have dark CO2 fixation capability (i.e., no requirement for photosynthesis), and most if not all also take up carbon skeletons from other organisms.   Thus, nature shows us that it is possible for higher plants to exist despite lacking intrinsic capability for photosynthesis.  Conceivably, the same was true in the Neoproterozoic.

It is possible that there are non-photosynthetic vascular plants in the fossil record, but I don't think there were any at Mistaken Point - simply because vascular plants did not evolve until ~150 million years after the Mistaken Point biota (in the Silurian/Devonian).  I don't know if there are any instances of algae that can live non-photosynthetically (if there are I would be interested...) but in any case I feel it would be unnecessarily complicated to suggest that Mistaken Point is the lone example from the fossil record of a diverse, abundant community of non-photosynthetic algae.  Because of their unusual morphology, it is always easy to argue that the Ediacara biota (and particularly the Mistaken Point biota) was something other than animals (a separate kingdom, lichen, fungi) but recent results on their community ecology has emphasized the striking similarities to modern epifaunal slope animal communities.

It seems unacceptably illogical to me for the paleobiology community to argue, on the one hand, that the volcanic ash of the Ediacaran at Mistaken Point was the key for the outstanding preservation of lifeforms on bedding planes, and on the other that the same ash destroyed every bit of the stalks and fronds of organisms envisaged to project upward from the seafloor.   The absence of any evidence in the volcanic tuff beds (some quite thick) for the fossils, and the presence of an abundance of non-stalked species -- such as the spindles -- leads me to consider that the majority of the fossils when living were actually either swimmers or at least buoyant well above the bottom.  Could it not be that instead of being seafloor dwellers, they were killed in surface water by the shock of volcanic explosion (just as fish are today) whereupon they settled flat to the bottom and were buried beneath the ash?   (Incidentally, cellulose and other plant polysaccharides have densities near 1.6 g/cm3, much denser than water.)

Holdfasts alone (without stems and fronds) are preserved on some ash-covered bedding planes, but it is much more common for the entire frond impression to be preserved.  Part of the reason that no fossils have been discovered within the tuff beds is that: 1) the later tectonic deformation created a much stronger cleavage in the ash beds than in the turbiditic strata, and 2) the ashfall tuff is only the (relatively) thin bottom portion of the entire ash bed - the bulk of the thick (30-50 cm) ash beds appear to be remobilized ash turbidites.  Holdfast discs from other localities have been found with stems projecting up into overlying beds.  It is also true that non-stalked species like spindles were abundant on some surfaces (the D and E surfaces especially), but other surfaces (e.g., Lower Mistaken Point) are nearly exclusively populated by stalked frondose species.   These community differences are actually meaningful (more on this at GSA and in an upcoming paper).  We feel that the Mistaken Point bedding surfaces represent census populations of the benthic communities - no planktonic/pelagic organisms are included.  The main line of evidence for this is taphonomic: that the organisms (even if they had a density of 1.6 g/cm3) wouldn't sink faster than the crystal tuff (which had a density of ~2.5-2.7 g/cm3).  In addition, the fact that the fossils with holdfasts have strong preferred orientations on all of the Mistaken Point surfaces (related to current directions) whereas the spindles are randomly oriented with no current accumulations (the equivalent to log-jams).  Although transport cannot be conclusively ruled out, the taphonomic evidence against it is quite robust.

Having seen what I did on the Mistaken Point trip, I see no reason for anyone taking a dogmatic stance about those organisms necessarily having a holdfast.   Certainly there was a circular structure associated with many of the fossils that could be (evidently has been extensively) interpreted as the base of a holdfast.  However, within an animal kingdom scenario, that structure might equally be interpreted as an early form of cephalization, with a long neck (rather than a "stalk") connecting the feeding 'head' to the swimming part.  In some of my photos there does appear to be a mouth-like opening in the circular structure, but none are fully persuasive because of the quality of preservation.    Within a plant kingdom scenario, morphogenesis as we understand it today is incredibly plastic.  That more or less spherical object at the end of the stalk could be a swelling created in response to polar transport of nutriment and growth regulators, at a time before genetic competence for either root or holdfast development had been attained.  It is reminiscent of the ball of callus cells that not uncommonly develops at the base of many species of plant cuttings after they've stood in water for a few weeks.  

The circular structure has been extensively interpreted as a holdfast because the frondose taxa are felled downcurrent from it, with an extremely strong preferred orientation, on every surface.  If these fossils were oriented after or during settling from a planktonic lifestyle then other organisms (like spindles) should also be similarly oriented.  This is not observed in any situations.   You mention that plant morphology can be quite plastic - the morphology of organisms at Mistaken Point was anything but plastic.  "Segmentation" is quite rigidly adopted in many taxa, and the morphology of those taxa is quite rigidly developed.  It is possible that the holdfast was (also) designed for nutrient transport in some kind of chemosynthetic/chemosymbiotic lifestyle, but the development in these communities of a complex tiering structure that is nearly identical to that of Phanerozoic suspension-feeding animal communities suggests that most if not all organisms at Mistaken Point were actually suspension-feeders.  There would not be as great of an impetus to tier even if they were deriving dissolved carbon or other nutrients from the water column.

These considerations take me back to what motivated me to participate in the field trip.  It seems to me that the uncertainties about Edicaran biota can only be resolved if specimens having excellent preservation, sufficient for microscopy, are found.  Unfortunately, Mistaken Point does not appear to be a source of such specimens, although it clearly is an outstanding site for both stratigraphic and paleoecological investigation, particularly as it is within an ecological reserve and protected by law.

with best wishes,

Rod Savidge, PhD
Professor, Physiology/Biochemistry
Forestry & Environmental Management
University of New Brunswick
Fredericton, NB E3B 6C2
tel. (506) 453 4919
fax (506) 453 3538

Hopefully this has satisfactorily addressed your points.  Best wishes,

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