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I've sent all of the articles surrounding the discussion of
Retallack's paper to Chris Nedin, because I thought he would be
interested. He was. He's asked me to submit the enclosed article on
his behalf. If you respond to this via the list, I will forward a
copy to Chris (unless it's clear that you already did). If you wish
to respond directly to Chris, his address is at the end of this
message.
Cheers!
--
Mickey Rowe (rowe@lepomis.psych.upenn.edu)
------------Start included message
The claim that all Ediacaran fossils exhibit the same taphonomy is
incorrect. There are, in the main, two different types of preservation,
which will be detailed below. Also, trace fossils show relief, indicating
an environmental cause for the relief, not a character implicit in the
Ediacaran organisms.
What follows was written a while ago for another forum, but provides some
background to the Ediacaran assemblage affinities arguement.
The fauna has now been found on all continents except Antarctica. However,
the most important sites are; Namibia, Newfoundland & MacKenzie Mountains,
Canada, the White Sea coast, Russia and the Flinders Ranges, South Australia.
One of the best localities and the place where the significance of the
fauna was first recognised is here in South Australia. The name Ediacara
comes from the site where the fossils were first found here. Fossils were
found in Namibia about 25 years ealier, but due to a mistake over the age
and the fact that the finds were published in foreign language journals
(German) their significance was not realized at the time.
Whilst the fauna has a world-wide distribution, it is important to note
that there are significant differences in the make-up of the fauna at
different localities. This is due, in the main, to environmental
conditions. The Australian and Russian forms are similar and the rocks are
indicative of a shallow water ecosystem. The Newfoundland forms are
decidedly different and the rocks are indicative of a deep water setting.
In South Australia, the fossils occur as depressions up into or extentions
down from the bottom of thin quartzite beds. The fossils were formed by the
covering of the muddy shallow sea floor and the organisms on it by
mantling, thin sand bodies. Those organisms which where able to support the
sand created depressions up into the overlying sand body (Fig 1). Those
Fig 1
Beast _
| -------------- --------------
____(^^^^)____ -> ____(^^^^)____SAND -> ____(^^^^)____SAND
MUD MUD |
_______________ ______________ Fossil_|
organisms which were either lying in or were forced into the mud by the
sand, allowed sand to fill in the void left as they decayed (to produce
downward extentions on the bottom of the sand body) (Fig 2)
Fig 2
Beast _
| -------------- --------------
_____ | ___ -> ____ ____SAND -> ____ ____SAND
MUD (____) MUD (____) (____)
_______________ ______________ |
Fossil-
The majority of fossils are of rounded forms, reminiscent of jellyfish and
in fact these were classified as jellyfish for a long time eg.
_Cyclomedusa_, _Mawsonites_. (up to 35 cm across) Other forms included
occasional 'sea-pen-like organisms (colonial octocorals) which appear very
similar to forms extant today (up to half a metre tall). A couple of
possible annelids such as the large sheet-like form _Dickinsonia_ which
looks like a flat pancake with segmentation, a gut and a definate head end
(up to 75 cm long); and _Spriggina_ which looks like a cross between a
bristle worm and a trilobite (5 cm). A possible arthropod is also present,
_Parvancorina_ a recent specimen of which shows gills and possibly legs (3
cm). A new Phylum appears to be represented by _Tribrachidium_, which as
it's name suggests is based on a tripartate body plan, but may well be some
form of lophophore (similar to brachiopods and bryozoa) (3 cm).
Until recently it was thought that the fauma was dominated by the motile,
free-swiming medusoids, which created a problem of preservation since
medusoids do not, as a general rule spend a lot of time on the sea bed in
the adult form. However recent work (Jenkins 1992) has shown that, whilst
medusoid forms are represented, the vast majority of rounded forms are the
anchors of sea-pens.
Modern sea-pens have a round, bulbous structure near the base of
the organism which is highly muscled. The organism uses this 'organ' to
burrow into the soft muddy sediment and then as an anchor to hold the
organism in place. During burial by the mantling sands, the stem of the
'sea-pen breaks off and the body of the 'sea-pen'. Since the 'sea-pen is
held up by hydrostatic pressure, the rip deflates the 'blade', the 'blade'
becomes mixed with the sand *thus diminishing its preservation potential*.
The bulb, on the other hand, is already buried. In life the bulb is filled
with water, so when the stem breaks away, the bulb fills with sand (Fig 3).
Since the underlying mud is approx. 80% water, as it dries
Fig 3
/^\
// \\
/ | | \
/ | | \
( | | )
( | | )
\ | | /
\ | | /
\ | | / (--)
\| |/ /
| | /
| | --> -------/ / ------------- -------------
| | SAND : : / SAND SAND
_____| |_____ _____ :/ _____ _____ _____ _____ _____
| | |:| * * * * * *
* * * : * _____________ |
* * * : : * |
* * * : * /|\ /|\ Fossil
MUD * MUD * | |
_____________ _____________ | Compaction |
out the thickness of the bed diminished to only a few cms, resulting in a
flattened, rounded outline to the fossil. The various classifications on
the 'medusoids' was due to surface ornamentation (ribs, concentric circles
etc.), these are now thought to be the manifestation of fibrous bands
within the 'holdfast' due to different degrees of decay before final
'molding'.
Thus the fauna has a decidedly benthonic bias, rather than being made up of
free swiming forms as previously thought.
There are two main theories as to the affinities of the Ediacara fauna.
One, put forward by Martin Glaessner is that most of the forms are related
to modern forms, if not direct precursers. The other, proposed by Dolf
Seilacher is that the Ediacaran fauna represents a unique bodyplan which
arose early in metazoan evolution and became extinct before the Cambrian
and thus all the forms within the fauna are members of a now extinct,
separate phylum - the Vendozoa, with no connection to modern forms- or even
Cambrian forms.
Recent finds of 'sea-pen-like organisms in the Burgess Shale, which are very
similar to Ediacaran forms appears to extend the range of such forms well
into the Cambrian. The form _Kimberella_ can be placed with confidence
within the Class Cubozoa (box jellyfish) Likewise the form _Chondroplon_
can be placed in the Suborder Chondrophorina. The form _Arkarua_ can be
placed in the Class Edioasteroidia.
Thus several groups within the Ediacaran fauna exist today and so the whole
fauna did not becone extinct. This is not to say that there are not some
unique forms, there are, but the idea that they are all unique is
oversteping things. My own opinion is that several groups of extant
organisma can be traced back to the Ediacaran fauna. However, the origin of
the metazoans is another matter. The Ediacaran fauna appears as a fully
intergrated ecosystem with some quite advanced forms (eg. the colonial
octocoral 'sea-pens'), so the question of origins has to be pushed back
even farther, probably IMHO to the late Proterozoic glaciation approx 900
mya. And IMHO body fossil evidence will never be found, since they occur in
meiofauna - too small to leave anything but chemical traces.
BTW the Newfoundland fauna are found in tubidites, covered by ash fallout,
settling through water. Such forms apparently existed in deep water and
were almost certainly heterotrophic, which might not mean much to most
people, but is another nail in the Vedozoa hypothesis.
The various elements of the Ediacara fauna are united by one common
character, none have any 'hard parts'. There is no evidence of
mineralisation in any fossil so far found. Thus the preservation of
essentially 'soft bodied' organisms presented something of a quandry,
especially as they are preserved in what is now quartzite. It was thought
that fossilization was due to a unique sedimentological facies, namely the
ripple-topped sands mantling muds and that the fossils were constrained by
the occurrance of this facies (eg. Mount 1989). However, the Ediacara
Member in the Flinders Ranges contains 5 separate facies, ranging from
thinly laminated silts to high energy, coarse sandstones, *each* of these
facies if fossiliferous to one degree or another.
Thus fossilization is not facies controlled, but occurs due to the
interplay of a number of factors (a common occurrance compared with a
simple 'cause and effect' answer - a fact which makes science interesting
and allowed the Commonwealth Postgraduate Research Scholarships office to
send me a meagre monthly allocation of drinking vouchers :-)).
Amongst the factors which allowed the preservation of the Ediacara fauna
are (in no particular order):
Collagen
bioturbation - the lack of
predation - the lack of
The ability to produce collagen is important because collagen is relatively
inert, strong and flexable. A collagen outer layer helped hold the organism
together. It also allowed the organism to retain it's shape when covered by
the mantling sand to produce the fossils. Also, since collagen was a
relatively new compound (it's synthesis was probably related to the
crossing of a threshold level of oxygen in the atmosphere), the micro-
community took a while to realize that collagen was a food source - see
predation below.
The lower level of oxygen in the atmosphere (compared with present levels),
plus the absence of suitable bodyplan made vertical burrowing vertially
unknown during this period. The lower oxygen levels meant that bodies had
to be kept small or thin - so because oxygen was adsorbed through the
surface of the organism (no lungs or gills - or more importantly a method
of delivering oxygen to the tissues from such organs [i.e. blood]).
Therefore, tissues had to be close to the surface in order to obtain oxygen
by simple diffusion. This meant thin bodies. There is very little
constraint to the size such organisms could reach, provided they stayed
thin, hence half metre long 'flat' worms. Flat, thin bodies are very bad at
burrowing, so no vertical burrows, which meant that any organism which was
buried was not disturbed, disrupted and ultimately distroyed by
bioturbation (as it common today).
Since there were no hard parts about, predation was well nigh impossible,
except possibly by disgorging some sort of dissolving fluid and sucking up
the resultant gastronomic soup. But, definately no chewing! Therefore, once
the organism shuffled off to join the ranks of the choir immortal, it's
mortal remains did just that - remained. They hung around on the surface,
undisturbed for a considerable period of time, waiting for the mantling
blanket of sand. As was mentioned before, collagen was probably still a
relatively novel compound at this time, so it was resistant to decay - i.e.
large numbers of collagen munching bacteria had yet to make an appearence.
However, these conditions did not last. An interrelated series of events
which included, the rise in oxygen levels, the aquisition of mineralization
capabilities, the rise of predation and the ability to produce a round
cross-sectional bodyplay (oxygen depended) condusive to burrowing, soon
demolished what was a pristene preservational environment.
The so called extinction of the Ediacara fauna is IMHO largely illusionary
for several reasons:
1) Of 7 cnidarian divisions represented in the Ediacaran fauna, 4 appear to
be anscestral to living taxa.
2) There is no close time control in respect of the supposed episode of
extinction.
3) the disappearence of the fauna is largely due to the closure of a
taphanomic or preservational 'window'.
4) The topmost facies of this period throughout the world indicate a
shallowing upward cycle, resulting in environments likely uncondusive to
preservation.
Refs
Jenkins, R.J.F. (1989) The 'supposed terminal Percambrian extinction event'
in relation to the Cnidaria. Memoir of the Association of Australasian
Palaeontologists, 8: 307-317 - a (very) few reprints available.
Jenkins, R.J.F. (1992) Functional and ecological aspects of Ediacaran
Assemblages. In Origin and Early Evolution of the Metazoa. J.H. Lipps and
P.W. Signor.131-177. Plenum Press, New York.
Mount, J.F. (1989) Re-evaluation of unconformities separating the
"Ediacaran" and Cambrian Systems, South Australia. Palaios, 4: 366-373
Chris
cnedin@geology.adelaide.edu.au, nedin@ediacara.org
-------------------------------------------------------------------
Many say it was a mistake to come down from the trees, some say
the move out of the oceans was a bad idea. Me, I say the stiffening
of the notochord in the Cambrian was where it all went wrong,
it was all downhill from there.
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