Chapter 8 Outline

The Precambrian Fossil Record

Evidence 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
Precambrian Microfossils
 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

Precambrian Macrofossils
 1.25 bya: Metaphytes
 0.6 bya: Ediacaran Fossils of Southern Australia
 Small shelly faunas (the Tomotian fauna)

The Precambiran Record
 The biosphere
 The atmosphere
 The lithosphere

Physical Features
 Banded Iron Formations
 Pyritic conglomerates
 Red Beds
The Proterozoic Diversification

Key Terms (see p.120 of text)

The Snowball Earth Theory

1.- Paleontological Features

Chronostratigraohic Divisions
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
  Eozoon canadense
  Stromatolites:What are stromatolies?
  Some examples of American Stromatolites
  Algae:What are algae?

  Ediacaran Fauna: Soft-bodied animals
        On the taxonomic status of Ediacaran Fossils
Skeletal fossils
        Cloudina: oldest known skeletal fossils
The nature of the terminal Proterozoic fossil record.

2.- Biological Features
    Evolutionary trends
  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


3.- Geological Features
    Paleogeography: Global events in the Proterozoic
        Glaciation in the Proterozoic
4.- Summary of Proterozoic Events

Acritarchs   (Late Precambrian - Quaternary) are marine, hollow, organic-walled unicellular vesicles. Their biological affinities are unclear. They are placed together with other palynomorphs due to their resistant cell wall.  Most of them are 0.02-0.150 mm across.

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.

Acritarchs 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.

Summary of Proterozoic Events

Paleoproterozoic: 2500-1600 mya
    2.6. bya: Stromatolites become widespread
    2.2 bya: Oldest known eukaryotes: Multicellular Aglae
                Simple Acritarch
    2.0 bya: Formation of Supercontinent Rodina,
                contained nearly all landmasses of the earth
    2.5 bya: Glaciation

Mesoproterozoic: 1600-1000 mya
    Prokaryote dominant
    Some Acritarch
    Maximun develolpment of stromatolites
    Banded Iron Formation

Neoproterozoic: 1000-543 mya
    More compex Acritarch
    Oldest animals
    800 mya: Glaciation: Snowball Earth
    Ediacara fauna

Microfossil types

Eozoon canadense

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

Fig. 1. The nature of the terminal Proterozoic fossil record.
(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 <>

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.

Evolution of Eukaryotes
Union of 2 prokaryotic cells
        Allow cells to derive energy from their food by respiration
        Evolved from 1 prokaryotic cell
        Site of photosynthesis
        Protozoan consumed, retained cyanobacterial cell

Stromatolites are laminated structures built mainly by cyanobacteria (sometimes known as blue-green bacteria or, less correctly, as blue-green algae). They are still found today, but were once much more common. They dominated the fossil record between about 2000 million and 1000 million years ago. Nowadays they are found mainly in saline lakes or hot spring environments, often in environmental niches that other organisms cannot tolerate.

The best example of living stromatolites is at Hamelin Pool, Shark Bay, Western Australia.

The 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: <>

Reconstruction 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  <>

PRECAMBRIAN:  Precambrian means: "before the Cambrian System."

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.

Division of the Precambrian based on Relative time: Chronostratigraphic Units


age of first life



Archaean “first”, "primary" or ancient
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.
MethanothermusMethanothermus sp. Courtesy Karl  Stetter.


 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 (pro-carry-oats) . 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 in mitochondria, 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 on Stramatolites <>

Important events in the history of life:
4.6 bya: Origin of Earth
3.5 bya: Prokaryoteds (bacteria dominate)
2.5 bya: Free oxygen accumulates in the atmosphere
1.5 bya: Eukaryotes: First nucleated cell
0.5 bya: Cambrian Explosion. First multicelular eukaryotes

VENDIAN also known as the Ediacaran  System:
The oldest fossils within KingdonAnimalia are Vendian-age 650 to 544 mya, are found at nearly 30 localaities around the world. The Ediacara Hills of Southern Australia, and the White Sea Region of Northern Russia are two of the more famous. The Ediaracan fossils where first discovered in 1946. They are regarded as 700 million years old it is the oldest fossil lagerstätte, named Ediacaran geological system. The preserved soft-bodied organisms, including similar forms now found worldwide are collectively referred to as Ediacaran biota. The Ediacaranis now a System in its own right, the first to be added to the Geological Time Scale in 120 years.  It extends from around 600 m.y. (the actual date has yet to be determined) to the beginning of the Cambrian at 542 m.y.  The International Commission of Stratigraphy, part of the International Union of Geological Scientists, made the decision in July of 2004.

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. <>

On the taxonomic status of Ediacaran Fossils: The Vendobionta
Ediacaran fossils were first discovered in the Flinders Range, South Australia in coarse sandstones lying beneath rocks with the first shelly fossils of Early Cambrian age. These are large, flat and carpet-like fossils. The initial work indicated that they were metazoans; that is, true animals -- medusoids, sea pens and annelid worms, possibly even arthropods and echinoderms -- and that they were soft-bodied precursors to animal groups found in Cambrian rocks. This traditional view was challenged by Dolf Seilacher, an iconoclastic German academic paleontologist, who declared that the Ediacaran fossils were not animals because none shows any evidence of having organs, muscles, mouth, anus, gut or legs. Instead he interpreted the Ediacarans as unique quilted and immobile organisms constructed as a series of fluid- or jelly-filled cells -- like air-mattresses -- that must have absorbed sunlight and nutrients directly from seawater through their skin. Seilacher suggested that these fossils represent a new kingdom of organisms called Vendobionta that became extinct before the beginning of the Cambrian.

Typically, the Vendian or Ediacaran fossils are preserved as thin impressions on bedding surfaces of fine to medium-grained sedimentary rocks. Ostensibly, these organisms were very thin, lacked any mineralized hard parts or well developed organs or organ systems, and had a quilt-like outer surface. Some uncertainty exists as to what groups of animals these fossils might represent, and, in fact, if they were ancestral to the multicellular organisms that appeared later in the Cambrian. The so-called Tommotian fauna (biota) appear near the end of theProterozoic, immediately preceding the Cambrian explosion. These small shelly animals were a prelude to the metazoans with hard exoskeletons that would rapidly diversify in the Cambrian.
Taken from: Ediacara Biota, ancestors of Modern Life or evolutionary dead end <>

Relative Time Scale for the Proterozoic

When began
My ago
1000 mya

2500 myra
This chart is modified from the International Commission on Stratigraphy