Geoscience Reference
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some thousands of individuals provides an
ideal opportunity to study the ecology
of this autochthonous community;
Clapham
et al
. (2003) used techniques
routinely applied to modern ecosystems,
including parameters of species richness,
organism abundance and biomass, and
diversity and evenness coefficients.
Bedding planes at Mistaken Point,
which record snapshots of the living
communities at the moment that they
were smothered by the volcanic ash,
preserve an average organism density of
60 fossils per square metre (36 fossils/m
2
[4/ft
2
] after retrodeformation;
23
), a
species richness between 3-12 taxa per
locality, and a biomass (expressed as a
percentage of total surface area) ranging
from 3.4% to 12.4%. Spindles comprise
30%, frondose and other upright forms
(
Charniodiscus
,
Charnia
, and
Thectardis
) comprise 55%, while the
remaining 15% includes
Bradgatia
,
Ivesia
, and
Hiemalora
. These values,
combined with estimated diversity and
evenness coefficients, all fall within the
range of modern slope communities and
suggest that ecological processes were
remarkably similar in the Precambrian to
those that operate today.
the Newfoundland species are endemic, it
is really only the frond-like forms, such as
Charnia
,
Charniodiscus
, and their holdfast,
Aspidella
, which are common to both
areas.
The South Australian assemblage is
dominated by a variety of simple disc-like
genera such as
Ediacaria
and
Cyclomedusa
(some of which may be synonyms of
Aspidella
), and also more complex discs
such as
Mawsonites
, with concentric rows
of lobate shapes getting larger from the
center outwards. Another enigmatic disc
is the trilobed
Tribrachidium
, which shares
its unusual threefold symmetry with
Triforillonia
from Mistaken Point.
Perhaps best known from South
Australia are the segmented forms such
as
Dickinsonia
and
Spriggina
. Initially
interpreted as flatworms and polychaete
worms respectively, it was then
pointed out that their segments do not
actually match up across the midline
and that they were therefore only
'pseudosegmented'. However, Jim Gehling,
of the South Australian Museum, has
countered this by showing that the
segments do match perfectly on the
rarely preserved ventral surfaces and
suggests that the mismatch on dorsal
surfaces is simply a result of squashing a
three-dimensional animal.
Other important forms from South
Australia include
Kimberella
, with fourfold
symmetry, now interpreted as an early
mollusk,
Arkarua
, with fivefold symmetry,
thought to be the earliest known
echinoderm, and
Paleophragmodictya
,
described as the earliest known sponge.
Another enigma is
Parvancorina
, thought
by some authorities to be an early
arthropod. The impression is that the
slightly younger Australian forms are a bit
more advanced than the fronds and
spindles that dominate assemblages at
Newfoundland, and have more in common
with some of the Phanerozoic animals
which were soon to replace them.
C
OMPARISON OF
M
ISTAKEN
P
OINT WITH OTHER LATE
P
RECAMBRIAN BIOTAS
Ediacara Hills, South Australia
The classic locality in the Flinders
Ranges, 300 km (190 miles) north of
Adelaide in South Australia (see Selden
and Nudds, 2004), is the most famous of
the various Ediacaran assemblages now
known from around the globe. Although
it has yet to be dated accurately, it is
possibly up to 10 million years younger
than the Mistaken Point occurrence, and
shows obvious differences in the
composition of the fauna (as well as in its
taphonomy and paleoecological setting -
see previous sections). Because many of
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