The Early Evolution of the Metazoa - in one easy post :-)
Part the First:-
The Ediacaran fauna (pronounced edi-akran) is a Precambrian
(Neoproterozoic) assemblage, which existed from about 610 million years ago
to approx 560 million year.*but see below*. [Note: the new timescale for
the Cambrian has the Period dominated by the Lower Cambrian and the whole
timespan has been reduced to approx. 40 million years; 545 - 505 million
years ago. Hence the Cambrian-Precambrian boundary is now at approx. 545
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
organisms which were either lying in or were forced into the mud by the
sand, allowed sand to fill in the void
| -------------- --------------
____(^^^^)____ -> ____(^^^^)____SAND -> ____(^^^^)____SAND
MUD MUD |
_______________ ______________ Fossil_|
left as they decayed (to produce downward extentions on the bottom of the
sand body) (Fig 2)
| -------------- --------------
_____ | ___ -> ____ ____SAND -> ____ ____SAND
MUD (____) MUD (____) (____)
_______________ ______________ |
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 750 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 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
/ | | \
/ | | \
( | | )
( | | )
\ | | /
\ | | /
\ | | / (--)
\| |/ /
| | /
| | --> -------/ / -------------
| | SAND : : : SAND
_____| |_____ _____ : _____ _____ _____
| | |:| * * *
* * * : * _____________
* * * : : *
* * * : * /|\ /|\
MUD * MUD * | |
_____________ _____________ | |
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 muscle bands 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
This later idea holds particular prominance in the US (eg. Stephen J Gould)
because [i think] it fits very neatly into the Punctuated Equilibrium
model. whereby you have a rapid evolutionary event followed by an
extinction event, then another evolutionary event (the Lower Cambrian).
However, close examination of the fossils shows that many of the forms do
indeed have a striking resemblance to Cambrian if not modern 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.
> Gould, in "Wonder Life", mentions an enigmatic group of
> fossils which are intermediate in age between the Ediacaran Fauna and the
> major groups which appear at the beginning of the Cambrian. This fauna is
> mostly disarticulated hard parts. Can you bring us up to date on this.
> Bruce Salem
Part the Second
These would be the tommotiids and the coeloscleritophorans. Both groups are
sub millimetric for the most part and come under the common name of "Small
The coeloscleritophorans are hollow calcareous sclerites with a scale- or
spine-shaped distal part and are morphologically very diverse. Amongst the
most important groups are the chancelloriids, which have composite,
star-shaped spicular rosettes; and the Halkieriids and Wiwaxiids, which
have elongated scale- and /or spine-shaped sclerites in tight intergration.
In all cases the sclerites formed a scaly or spiney armour covering the
body surface. They were hollow with a calcareous wall. They did not grow by
simple accretion, but were shed or augmented by interpolation. the group
arose near the Cambrian-Precambrian transition, diversified in the Lower
Cambrian and became extinct in the Late Cambrian.
Early work on this group was a mess, since it was first thought that each
individual sclerite represented a single organism and so a plethora of
species ware named, numbering in the thousands, before it was realised that
each individual organism has many sclerites linked together, they fell
apart as the organism decayed (kind of like you dying and leaving behind
only your finger- and toenails. Then someone coming along and classifying
each and every nail as a new species). However, today the group is
important in Lower Cambrian correlations and probably represent the outer
coverings of moluscs (?Halkieriids) and annelids (Wiwaxia - see Butterfield
1990). The intact halkierid has now been found (Conway Morris & Peel 1990)
and is simila to the Midle Cambrian form Wiwaxia from the Burgess Shale. It
is a bilaterally symmetrical metazoan with various zones of differently
shaped elongated sclerites including, cultrate, siculate and plamate forms.
however, totally unexpectedly, the animal has a pair of shells at the
anterior and posterior ends, looking very much like monoplacophoran shells
(a primative molusc).
The Tommotiids, on the other hand, are phosphatic (as compared with
calcareous). They are conical sclerites which grew by internal basal
secretion. These possibly repredsent another mode of preservation with
growth not unlike the fingernail.
Hope this helps,
| email@example.com | "How can Nedin be trusted" |
| Dept. of Geology & Geophysics | C Wieland Director, |
| University of Adelaide | Creation Research Foundation, |
| South Australia 5005 | Queensland Australia |
References for the vaguely interested
Part the First
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.
McMenamin, M.A.S. & McMenamin, D.L.S. (1990) The Emergence of Animals: The
Cambrian Breakthrough. Columbia University Press, New York.
Part the Second
Bengston, S. & Conway Morris, S. (1984) A comparitive study of Lower
Cambrian _Halkieria_ and Middle Cambrian _Wiwaxia_. Lethaia, 17: 307-329.
Bengston, S. & Missardzheusky, V.V. (1981) Coelosclerictophora - a major
group of enigmatic Cambrian metazoans. USGS Open-file Report 81-743, 19-21.
Butterfield, N.J. (1990) a reassessment of the enegmatic Burgess Shale
fossil _Wiwaxia corrugata_ (Mathew) and its relationship to the polychete
worm _Canida spinosa_. Paleobiology, 16(3): 287-303.
Conway Morris, S. (1987) The search for the Cambrian-Precambrian boundary.
American Scientist, 75: 156-167.
Conway Morris, S. & Chen Menge (1990) Tommotiids from the Late Cambrian of
South China. Journal of Paleontology, 64: 169-184.
Conway Morris, S. & Peel, J.S. (1990) Articulated halkieriids from the
Lower Cambrian of North Greenland. Nature, 345: 802-805 - Includes specy
pic on cover.