Third and last period of the Neoproterozoic Era
The
Ediacaran
(
EE
-dee-
AK
-?r-?n,
ED
-ee-
)
[3]
is a
geological period
of the
Neoproterozoic
era
that spans 96 million years from the end of the
Cryogenian
period at 635
Mya
to the beginning of the
Cambrian
period at 538.8 Mya.
[4]
It is the last period of the
Proterozoic
eon
as well as the last of the so-called "
Precambrian
supereon", before the beginning of the subsequent Cambrian period marks the start of the
Phanerozoic
eon, where recognizable
fossil
evidence of
life
becomes common.
The Ediacaran period is named after the
Ediacara Hills
of
South Australia
, where
trace fossils
of a diverse community of previously unrecognized lifeforms (later named the
Ediacaran biota
) were first discovered by geologist
Reg Sprigg
in 1946.
[5]
Its status as an official geological period was ratified in 2004 by the
International Union of Geological Sciences
(IUGS), making it the first new geological period declared in 120 years.
[6]
[7]
[8]
Although the period took
namesake
from the Ediacara Hills in the
Nilpena Ediacara National Park
, the
type section
is actually located in the bed of the Enorama Creek
[9]
within the Brachina Gorge
[10]
in the
Ikara-Flinders Ranges National Park
, at
31°19′53.8″S
138°38′0.1″E
/
31.331611°S 138.633361°E
/
-31.331611; 138.633361
, approximately 55 km (34 mi) southeast of the Ediacara Hills fossil site.
The Ediacaran marks the first widespread appearance of complex
multicellular
fauna
following the end of the Cryogenian global
glaciation
known as the
Snowball Earth
. The relatively sudden
evolutionary radiation
event, known as the
Avalon Explosion
, is represented by now-extinct, relatively simple
soft-bodied
animal
phyla
such as
Proarticulata
(
bilaterians
with simple
articulation
, e.g.
Dickinsonia
and
Spriggina
),
Petalonamae
(
sea pen
-like animals, e.g.
Charnia
),
Aspidella
(radial-shaped animals, e.g.
Cyclomedusa
) and
Trilobozoa
(animals with
tri-radial symmetry
, e.g.
Tribrachidium
). Most of these organisms appeared during or after the Avalon explosion 575 million years ago and died out during the
End-Ediacaran extinction
event 539 million years ago. Forerunners of some modern animal phyla also appeared during this period, including
cnidarians
and early bilaterians such as
Xenacoelomorpha
, as well as
mollusc
-like
Kimberella
. Hard-bodied organisms with
mineralized
shells
or
endoskeletons
, which can be
fossilized
and preserved, were yet to evolve and would not appear until the superseding
Cambrian Explosion
some 35 million years later.
The supercontinent
Pannotia
formed and broke apart by the end of the period. The Ediacaran also witnessed several
glaciation events
, such as the
Gaskiers
and
Baykonurian glaciations
. The
Shuram excursion
also occurred during this period, but its glacial origin is unlikely.
Ediacaran and Vendian
[
edit
]
The Ediacaran Period overlaps but is shorter than the
Vendian
Period (650 to 543 million years ago), a name that was earlier, in 1952, proposed by Russian geologist and
paleontologist
Boris Sokolov
. The Vendian concept was formed stratigraphically top-down, and the lower boundary of the Cambrian became the upper boundary of the Vendian.
[11]
[12]
Paleontological substantiation of this boundary was worked out separately for the
siliciclastic
basin (base of the
Baltic
Stage of the
Eastern European Platform
[13]
) and for the
carbonate
basin (base of the
Tommotian
stage of the
Siberian Platform
).
[14]
The lower boundary of the Vendian was suggested to be defined at the base of the
Varanger
(
Laplandian
)
tillites
.
[12]
[15]
The Vendian in its type area consists of large subdivisions such as Laplandian,
Redkino
,
Kotlin
and
Rovno
regional stages with the globally traceable subdivisions and their boundaries, including its lower one.
The Redkino, Kotlin and Rovno regional stages have been substantiated in the type area of the Vendian on the basis of the abundant organic-walled
microfossils
, megascopic algae,
metazoan
body fossils and
ichnofossils
.
[12]
[16]
The lower boundary of the Vendian could have a
biostratigraphic
substantiation as well taking into consideration the worldwide occurrence of the Pertatataka assemblage of giant acanthomorph
acritarchs
.
[15]
Upper and lower boundaries
[
edit
]
The Ediacaran Period (c. 635?538.8 Mya) represents the time from the end of global
Marinoan glaciation
to the first appearance worldwide of somewhat complicated trace fossils (
Treptichnus pedum
(Seilacher, 1955)).
[6]
Although the Ediacaran Period does contain soft-bodied
fossils
, it is unusual in comparison to later periods because its beginning is not defined by a change in the fossil record. Rather, the beginning is defined at the base of a chemically distinctive
carbonate
layer that is referred to as a "
cap carbonate
", because it caps glacial deposits.
This bed is characterized by an unusual depletion of
13
C
that indicates a sudden climatic change at the end of the
Marinoan
ice age
. The lower
global boundary stratotype section (GSSP)
of the Ediacaran is at the base of the cap carbonate (Nuccaleena Formation), immediately above the Elatina
diamictite
in the Enorama Creek section, Brachina Gorge, Flinders Ranges, South Australia.
The
GSSP
of the upper boundary of the Ediacaran is the lower boundary of the Cambrian on the SE coast of Newfoundland approved by the International Commission on Stratigraphy as a preferred alternative to the base of the
Tommotian
Stage in
Siberia
which was selected on the basis of the ichnofossil
Treptichnus pedum
(Seilacher, 1955). In the history of stratigraphy it was the first case of usage of bioturbations for the System boundary definition.
Nevertheless, the definitions of the lower and upper boundaries of the Ediacaran on the basis of chemostratigraphy and
ichnofossils
are disputable.
[15]
Cap carbonates generally have a restricted geographic distribution (due to specific conditions of their precipitation)
[
vague
]
and usually siliciclastic sediments laterally replace the cap carbonates in a rather short distance but cap carbonates do not occur above every tillite elsewhere
[
clarification needed
]
in the world.
The C-isotope chemostratigraphic characteristics obtained for contemporaneous cap carbonates in different parts of the world may be variable in a wide range owing to different degrees of secondary alteration of carbonates, dissimilar criteria used for selection of the least altered samples, and, as far as the C-isotope data are concerned, due to primary lateral variations of δ
l3
C
carb
in the upper layer of the ocean.
[15]
[18]
Furthermore,
Oman
presents in its stratigraphic record a large negative carbon isotope excursion, within the Shuram
[19]
Formation that is clearly away from any glacial evidence
[20]
strongly questioning systematic association of negative δ
l3
C
carb
excursion and glacial events.
[21]
Also, the
Shuram excursion
is prolonged and is estimated to last for ~9.0 Myrs.
[22]
As to the
Treptichnus pedum
, a reference ichnofossil for the lower boundary of the Cambrian, its usage for the stratigraphic detection of this boundary is always risky, because of the occurrence of very similar trace fossils belonging to the Treptichnids group well below the level of
T. pedum
in
Namibia
,
Spain
and
Newfoundland
, and possibly, in the
western United States
. The stratigraphic range of
T. pedum
overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain.
[15]
[23]
Subdivisions
[
edit
]
The Ediacaran Period is not yet formally subdivided, but a proposed scheme
[24]
recognises an Upper Ediacaran whose base corresponds with the
Gaskiers glaciation
, a Terminal Ediacaran Stage starting around
550
million years ago
, a preceding stage beginning around 575 Ma with the earliest widespread
Ediacaran biota
fossils; two proposed schemes differ on whether the lower strata should be divided into an Early and Middle Ediacaran or not, because it is not clear whether the Shuram excursion (which would divide the Early and Middle) is a separate event from the Gaskiers, or whether the two events are correlated.
Absolute dating
[
edit
]
The
dating
of the rock type section of the Ediacaran Period in South Australia has proven uncertain due to lack of overlying igneous material. Therefore, the age range of 635 to 538.8 million years is based on
correlations
to other countries where dating has been possible. The base age of approximately 635 million years is based on
U?Pb
(
uranium
?
lead
) and
Re?Os
(
rhenium
?
osmium
) dating from Africa, China, North America, and Tasmania.
[25]
[26]
[27]
[28]
[29]
Biota
[
edit
]
The fossil record from the Ediacaran Period is sparse, as more easily fossilized hard-shelled animals had yet to evolve. The Ediacaran biota include the oldest definite
multicellular organisms
(with specialized tissues), the most common types of which resemble segmented worms, fronds, disks, or immobile bags.
Auroralumina
was a
cnidarian
.
[31]
[32]
Most members of the Ediacaran biota bear little resemblance to modern lifeforms, and their
relationship
even with the immediately following lifeforms of the
Cambrian explosion
is rather difficult to interpret.
[33]
[34]
More than 100
genera
have been described, and well known forms include
Arkarua
,
Charnia
,
Dickinsonia
,
Ediacaria
,
Marywadea
,
Cephalonega
,
Pteridinium
, and
Yorgia
. However, despite the overall enigmaticness of most Ediacaran organisms, some fossils identifiable as hard-shelled agglutinated
foraminifera
(which are not classified as animals) are known from latest Ediacaran sediments of western Siberia.
[35]
Sponges
recognisable as such also lived during the Ediacaran.
[36]
Four different biotic intervals are known in the Ediacaran, each being characterised by the prominence of a unique ecology and faunal assemblage. The first spanned from 635 to around 575 Ma and was dominated by acritarchs known as
large ornamented Ediacaran microfossils
.
[37]
The second spanned from around 575 to 560 Ma and was characterised by the Avalon biota. The third spanned from 560 to 550 Ma; its biota has been dubbed the White Sea biota due to many fossils from this time being found along the coasts of the
White Sea
. The fourth lasted from 550 to 539 Ma and is known as the interval of the Nama biotic assemblage.
[38]
There is evidence for
a mass extinction
during this period from early animals changing the environment,
[39]
dating to the same time as the transition between the White Sea and the Nama-type biotas.
[40]
[41]
Alternatively, this mass extinction has also been theorised to have been the result of an
anoxic event
.
[38]
Astronomical factors
[
edit
]
The relative proximity of the Moon at this time meant that
tides
were stronger and more rapid than they are now. The day was 21.9 ± 0.4 hours, and there were 13.1 ± 0.1 synodic months/year and 400 ± 7 solar days/year.
[42]
Documentaries
[
edit
]
A few English language documentaries have featured the Ediacaran Period and biota:
- The Time Traveller's Guide To Australia
(2012,
ABC Science
; Part 1 of 4).
[43]
- The Geological History of Canada
, as part of
The Nature of Things
series, CBC-SRC; 2011; Eastern Canada.
- The first episode of a BBC documentary titled
Life on Earth
, with
David Attenborough
as narrator.
- Another documentary narrated by David Attenborough titled
First Life
featuring
Charnia
,
Dickinsonia
,
Spriggina
,
Funisia
, and
Kimberella
animated in
CGI
.
- In our time
- Ediacara Biota
, BBC, 9 July 2009
See also
[
edit
]
References
[
edit
]
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- ^
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Celebrating 50 years of ABC Science
Retrieved 18 March 2023.
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