Chapter 8 Outline
The Precambrian Fossil Record
for Precambrian Life
Morphological fossils -black chert
Chemical fossils –C12/C13 ratios
Pristane and Phytane: stable geologically stable organic molecules
Products of decomposition of chlorophyll
3.5 bya: Warrowoonda Group, western Australia
3.4 bya: Fig Tree Formation, Southern Africa
2.1 bya: Gunflint Chert, Canada
2.0 to 1.8 bya: Oldest Eukaryote
0.75 bya: Bitter Springs Formation, Central Australia
1.25 bya: Metaphytes
0.6 bya: Ediacaran Fossils of Southern Australia
Small shelly faunas (the Tomotian fauna)
Banded Iron Formations
The Proterozoic Diversification
Key Terms (see p.120 of text)
Time interval: 2500 mya to 543 mya
RelativeTime Scale for the Proterozoic
Main Fossil Types
Microfossils of the Paleoproterozoic ( 2,100-million-year-old) Gunflint chert of southern Canada.
Filamentous microfossils of the Neoproterozoic ( 850-million-year-old) Bitter Springs chert of central Australia
Stromatolites:What are stromatolies?
Some examples of American Stromatolites
Algae:What are algae?
Fauna: Soft-bodied animals
On the taxonomic status of Ediacaran Fossils
Cloudina: oldest known skeletal fossils
The nature of the terminal Proterozoic fossil record.
Cyanobacteria flurished: Stromatolites
Stromatolites in the hyper-saline water of Hamelin Pool at the base of Shark Bay in W.A.
Eukaryotes abundant: Early eukaryotes
Major events leading from prokaryotes to multicelluar animals
Algae: Have a look at some green algae
Grypania a coiled muticelluar algae
Paleogeography: Global events in the Proterozoic
Glaciation in the Proterozoic
4.- Summary of Proterozoic Events
The term Acritarchs was coined by Evitt in 1963, to refer to microfossils "of uncertain origin", they are an artificial group. The group includes any small (most are between 20-150 microns across), organic-walled microfossil which cannot be assigned to a natural group. They are characterised by varied sculpture, some being spiny and others smooth. They are believed to have algal affinities, probably the cysts of planktonic eukaryotic algae. They are valuable Proterozoic and Palaeozoic biostratigraphic and palaeoenvironmental tools.
Acritarchs are extant but little is known about the organism which produces them. It is generally accepted that they are probably the resting cysts of marine phytoplankton and therefore represent only one stage of a multi-stage lifecycle. The gross morphology of acritarchs, a single organic sac or hollow vesicle, and their size, generally between 15 to 80 microns, suggests derivation from a unicellular organism. Many show simple excystment structures which may be divided into two groups: linear sutures which vary in position and shape, and pylomes which are complete small circular holes in the vesicle which may have a plug which fits in the hole called an operculum. These features are strongly suggestive of a dinoflagellate-like host organism. The fact that they are only found in rocks of marine origin, their great age and other preservational associations allows us to be almost sure they are wholly marine.
The earliest palynomorphs which we now classify as acritarchs are probably those reported by White in 1862 from the Ordovician to Devonian of New York. In the early 1930's Eisenack, working on material from the Baltic, re-assigned many species to the acritarch group and came to regard them (as we do now) as being derived from phytoplankton. It was Downie, Evitt and Serjeant however who tidied up the classification, providing a usable system, even though we are still uncertain of the groups true affinities.
The oldest known Acritarchs are recorded from shales of Palaeoproterozoic (1900-1600 Ma) age in the former Soviet Union. They are stratigraphically useful in the Upper Proterozoic through to the Permian. From Devonian times onwards the abundance of acritarchs appears to have declined, whether this is a reflection of their true abundance or the volume of scientific research is difficult to tell.
Natural classifications of acritarchs have not often been attempted nor accepted. The artificial scheme introduced by Downie, Evitt and Serjeant has been widely accepted and serves the purposes of most palynologists so there is little incentive to change it. Acritarchs are therefore divided into the following groups (most common first) based on their morphology.
Acanthomorphs have spherical bodies with spines which usually open into the body.
Polygonomorphs have a body-shape defined by the number and position of spines, they are often triangular or square in outline.
Netromorphs have a fusiform body with one or more spines.
Diacromorphs have spherical to ellipsoidal bodies with ornament confined to the poles.
Prismatomorphs have prismatic to polygonal bodies the edges of which form a flange or crest which may be serrated.
Oomorphs have an egg shaped body with ornament confined to one pole.
Herkomorphs have a roughly spherical body divided into polygonal fields, rather like a football.
Pteromorphs have a roughly spherical central zone often compressed, surrounded by a flange or wing lamella which may be sustained by radial folds or processes, they resemble under the light microscope a fried egg!
Sphaeromorphs have simple spherical morphology.
In summary: Acritarchs are spherical microfossils that look very much like the resting stage cysts of dinoflagellates and other algae. Some have spines; some are completely bare; but all lack some of the characteristic features of dinoflagellate cysts. These enigmatic fossils may belong to many different groups. Acritarchs first appeared in the fossil record in the Precambrian about 1.8 billion years ago, and they are still around today.
are organic-walled cysts of unicellular protists that cannot be assigned
to any known group of organisms. Most acritarchs are probably the resting
cysts of marine eukaryotic phytoplankton. Some acritarchs are thought to
be dinoflagellate cysts but lack the requisite morphology to make a positive
attribution. Others, however, can be confidently assigned to the chlorophytes
(green algae), but for convenience, are still commonly included in the
acritarchs. Thus, acritarchs are a heterogeneous, polyphyletic collection
of organic-walled microfossils of unknown or uncertain origin. Acritarchs
vary in size from < 10 microns to more than 1 mm, but the majority of
species range from 15 to 80 microns. Because of their small size, abundance
and diversity, as well as widespread distribution, acritarchs are very
useful in biostratigraphic correlation, as well as paleobiogeographic and
paleoenvironmental studies. Acritarchs are found throughout the geologic
column but were most common during the Late Proterozoic and Paleozoic.
Because they represent the fossil record of the base of the marine food
chain during the Proterozoic and Paleozoic, acritarchs played an important
role in the evolution of the global marine ecosystem.
2.6. bya: Stromatolites become widespread
2.2 bya: Oldest known eukaryotes: Multicellular Aglae
2.0 bya: Formation of Supercontinent Rodina,
contained nearly all landmasses of the earth
2.5 bya: Glaciation
Maximun develolpment of stromatolites
Banded Iron Formation
More compex Acritarch
800 mya: Glaciation: Snowball Earth
microfossils of the Neoproterozoic (
850-million-year-old) Bitter Springs chert of central Australia
1. The nature of the terminal Proterozoic fossil
(A) Ediacaria, a radially symmetrical cast preserved on the underside of a sandstone bed, Rawnsley Quartzite, South Australia.
(B) Macroscopic alga preserved as a carbonaceous compression in shales of Doushantuo Formation, China.
(C) Calcified fossils in limestone of the Nama Group, Namibia.
(D) Pteridinium, a frondose Ediacaran fossil consisting of three vanes built of repeating units (two visible in specimen) that are joined along a central axis.
(E) Phospatized animal egg and early cleavage-stage embryo, Doushantuo Formation.
(F) Simple trace fossils of bilaterian animals, Rawnsley Quartzite. Bar = 2.5 cm for (A), 3 mm for (B), 1.5 cm for (C) and (D), 250 µm for (E), and 2 cm for (F).
Source: Early Animal Evolution: Emerging Views from Comparative Biology and Geology by Andrew H. Knoll and Sean B. Carroll <http://cas.bellarmine.edu/tietjen/Ecology/early_animal_evolution.htm>
What are algae?
The term 'algae' is used for some lower plants and many, often unrelated groups of microorganisms that are able to perform photosynthesis.
Photosynthesis (converting light energy into chemical energy) is performed in parts of the cell called chloroplasts. They can be found in different shapes and colours and in many different organisms. Not all these organisms are green. Diatoms, Chrysophytes and dinoflagellates have yellow to brown chloroplasts. There are brown algae (Phaeophyta), red algae (Rhodophyta) and many other groups of unicellular algae in many shades of green. The blue green Cyanobacteria also photosynthesize.
A very diverse
group of freshwater algae are the Chlorophytes or Green algae. Based on
the compounds of the photosynthetic pigments and several other characteristics
they seem closest related to plants.
The best example of living stromatolites is at Hamelin Pool, Shark Bay, Western Australia.
bacteria precipitate or trap and bind layers of sediment to make accretionary
structures, which can be stratiform, domical, conical or complexly branching.
They can range in size from smaller than a little finger to larger than
a house. Some branching stromatolites resemble modern corals.
Some, but not all, stromatolitic bacteria are photosynthetic. Such organisms were probably largely responsible for the eventual oxygenation of Earth's atmosphere. There are long-standing controversies about the participation of organisms in the formation of stromatolite-like structures, as well as about the definition of the term stromatolite itself. Stromatolites have been defined as "morphologically circumscribed accretionary growth structures with primary
lamination that is, or may be, biogenic". A more recent term, microbialite, refers to both laminated and unlaminated structures of "undisputed microbial origin", and thus includes biogenic stromatolites, as well as the structures known as thrombolites, such as those found in Lake Clifton, near Mandurah, Western Australia.
Living stromatolites at Shark Bay, Western Australia (see image) showing the laminated structure of a single column and the microscopic cyanobacteria that construct them. Each cyanobacterial filament is about 100th of a millimetre in width. These organisms are rarely preserved in fossil stromatolites, although some have been discovered in chert (silica-rich rock) at other localities in the Pilbara that are about the same age as the Trendall locality. Source: <http://www.doir.wa.gov.au/5257.aspx>
of the supercontinent of Rodinia at 600 million years ago. The triangles
represent coeval glacial deposits (Varangerian), which forms a continuous
belt from the Scandinavia to Australia. "?" means deposit with supposed
but not confirmed glacial influence, including Southern Brazil and Uruguay.
Image Source: on-line paper by Toni Eerola <http://www.helsinki.fi/hum/ibero/xaman/articulos/2001_01/eerola.html>
During the eighteenth century geologists began mapping the rock sequences of the earth's crust. They frequently found a basement complex made of igneous and metamorphic rocks beneath the lowest sedimentary layers. These were called the "Primitive" or "Primary", although the term "Primary Era' later came to be applied to the oldest sedimentary stage (later to be called the Paleozoic).
In 1835 the English geologist Adam Sedgwick used the name "Cambrian" for the oldest sedimentary strata. Thereafter the underlying rocks were term Precambrian - "before the Cambrian". Thus, the Precambrian was originally defined as the era that predated the appearance of life in the Cambrian System.
In the last few decades geologists have found that there are some hard-to-discern fossils in some Precambrian rocks, so this period was now also known as the Cryptozoic Eon or "hidden life" (from the words "crypt" = "hidden," and "zoon" = "life"). During the twentieth century the term "Cryptozoic" - age of hidden life" - was used to designate this period, whereas the Phanerozoic - "age of obvious (or revealed) life" - was used for those periods from which fossils of multicellular organisms are known (i.e. the Cambrian period to the present-day). Although the latter term "Cryptozoic" is still in use by some geologist, it formally disappeared in favor of the older and well established term Precambrian. This old, but still common term has not a formal status in the subdivisions of the geological time scale; however, we still use the term Precambrian as it was originally used: to refer to the whole period of earth's history before the formation of the oldest rocks with recognizable fossils in them.
The Precambrian covers almost 90% of the entire history of the Earth, from 4,500 to about 443 mya (million years ago). The Precambrian would be regarded as a first order division of the geological time scale, that is, an Eonothem; however, as stated above, it is not considered as a formal division of the geologic time scale. Instead the Precambiran time is divided into three major parts that are considered as first order divisions: the Priscoan Eonothem (also known as the Hadean), the Archean Eonothem and the Proterozoic Eonothem.
age of first life
Priscoan (Hadean): the time when the geosphere was still forming and life had not developed. Hadean referring to the hellish conditions of the very early Earth.
It is believed that life
on earth made its appearance in the seas during Archaean . The first life
is believed to be the Eubacteria
(i.e., bacteria prokaryotic organisms). The most accepted
theory is that the Eubacteria are the ancestors of the members
of the Domain Arachaea which includes organisms that can exist
in extremely hostile environments such as thermal vents and hypersaline
water. However, not all Archaeans are extremophiles , and,
in fact, this domain is extremely diverse, and only recently being studied
using genomic and proteomic methods. The earliest bacteria obtained energy
through chemosynthesis (ingestion of organic molecules). They represent
the oldest fossils that go back to about 3500 mya , and are known
as bacterial microfossils. Archaeans are single-celled creatures
that join bacteria to make up a category of life called the Prokaryotes
Prokaryotes' genetic material, or DNA, is not enclosed in a central cellular
compartment called the nucleus. Bacteria and Archaea
are the only prokaryotes. All other life forms are Eukaryotes (you-carry-oats)
creatures whose cells have nuclei. (Note: viruses are not considered true
cells, so they don't fit into either of these categories.). However, while
archaeans resemble bacteria and have some genes that are similar
to bacterial genes, they also contain other genes that are more like what
you'd find in eukaryotes. Furthermore, they have some genes that aren't
like any found in anything else. View
the difference between Prokaryots and Eukaryotes
The hypothesized process by
which prokaryotes gave rise to the first eukaryotic cells is known as endosymbiosis
The term was coined by Margulis: Lynn Margulis (born 1938) is a biologist
and a professor at the University of Massachusetts Amherst. In 1967 she
proposed a contentious new hypothesis which became her most important scientific
contribution as the endosymbiotic theory of the origin of mitochondria
as separate organisms that long ago entered a symbiotic relationship with
eukaryotic cells through endosymbiosis. After the proposal of the endosymbiotic
theory, Margulis predicted that if organelles were prokaryotic symbionts,
then the organelles will have their own DNA that would be different from
the DNA of the cell. This prediction was actually proven in the 1980's
centrioles, and chloroplasts.
These earliest fossils display
what appear to be chemical signs of delicate chains of microbes that appear
exactly like living blue-green algae (known as cyanobacteria ). For
billions of years these bacteria formed extensive slimy carpets in shallow
coastal waters, and before the end of Achaean about 2.5 bya
they had also formed a thin crust on land. The structures that these bacteria
formed are know as stromatolites,
these accretionary growth structures produced by the prokaryotes,
and also possibly Arachaeans and primitive Eukaryotes, became increasingly
abundant during the Archaean, a fact of critical importance to the
later evolution of life. However, an alternate hypothesis postulates that
eukaryotes may have appeared in the late Archaea. Stromatolitic
structures span the Precambrian and extend to modern time, though they
are currently limited to several isolated environments.More information
The oldest diverse (and abundant) representatives
of the Ediacaran soft-bodied macrobiota are preserved beneath volcanic
ash layers 575-565 Ma on the Avalon Peninsula of eastern Newfoundland,
Atlantic Canada. They first appear ~5 Myr after the Gaskiers glaciation
in the same succession. Many of the forms were frond-like, with holdfasts
attached to the sea-floor like modern “sea pens”. The Ediacaran biota includes
cnidarian sponges and one form, Kimberella, found in younger (550 Ma) Ediacaran
strata on the White Sea Coast of Arctic Russia, is generally accepted to
be a stem group mollusc. But the once heretical view that few of the Ediacaran
organisms were animals appears to be gaining ground. They flourished up
until the Ediacaran-Cambrian boundary (442 Ma), when they suddenly disappear
from the record in the oldest-known mass extinction. <www.snowballearth.org/week12.html>
Relative Time Scale for the Proterozoic
The importance of volcanic activity/plate tectonics in the evolution of life on earth
Name three features that allows to speak of an amazing earth
Snowball theory: the role it played in the evolution of life in the Precambrian
Name the super Precambrian super continent
Why the Pacific is called the mother of all oceans?
3 Trillion tons of water evaporate each day
Ocean floor is complex: mountain ranges, and trenches
Coral reefs: geology and biology: Why?
Cont shelf drop deeper into the ocean. 650 feet
Cont slope 5000 feet to the deepest part
Ocean floor- abyssal plain: Canyons, mid-ocean ridge 40K miles across
the world oceans- hydrothermal vents: water is heated from molten lava-
creates the smokers.
Home of a number of organisms w/o sunlight. How life was born under brutal condition of the primordial oceans?
Where oceans came from?
4.6 mya coalesced large asteroids and comets: Molten earth first continents began to form.
3.8 mya emerged as an ocean plan
Oceans of the past were Dense, acidic corrosive
Life appears under volcanic vents
3.5 mya: hot lava still under the oceans
But in shallow water stromatolites developed: colonies of bacteria: Photosynthetic
More O into the world, stay in the surface absorbed by the rocks
But later it went to the surface to the atmosphere, it was poisonous
Something went wrong; Cosmic snowball, 2.2. bya
O2 destroyed the green house gas: Methane- world cooled more cooling more ICE Run away ALBEDO
Sea ice: to the equator, engulfed the tropics. Like Europa, or Io. To a depth of ½ mile
Plate tectonics began- life in the oceans changed
2.0 bya: Continents- endless oceans, bacteria,
1.0 bya multi cellular organisms appears
super continent of RODINIA
600: PANASIA super continent
Another snowball appeared
540 mya shallow seas-CAMBRIAN EXPLOSSION :Predator-pray