The Earth Through Time, 8e

Eighth Edition
by Harold L. Levin

Chapter 9 - page 6

The Proterozoic: Dawn of a More Modern World

Proterozoic Life

Life at the beginning of the Proterozoic was similar to that in the Archean:

Other forms of life appeared during the Proterozoic, including:

Microfossils of the Gunflint Chert

First definitive Precambrian fossils to be discovered (1953) were in the 1.9 by old Gunflint Chert, NW of Lake Superior (Paleoproterozoic).

The fossils are well-preserved, abundant and diverse and include:

Gunflint fossil organisms resemble photosynthetic organisms, and the rock containing these organisms contains organic compounds that are regarded as the breakdown products of chlorophyll.

The Gunflint Chert organisms altered the composition of the atmosphere by producing oxygen.

Diagrams of organisms in the Gunflint Chert.
Diagrams of organisms in the Gunflint Chert.
A = Eoastrion ( = dawn star), probably iron- or magnesium-reducing bacteria
B = Eosphaera, an organism or uncertain affinity, about 30 micrometers in diameter
C = Animikiea (probably algae)
D = Kakabekia, an organism or uncertain affinity

The Rise of Eukaryotes

The appearance of eukaryotes is a major event in the history of life.
Eukaryotes have the potential for sexual reproduction, which increases variation through genetic recombination.

Genetic recombination provides greater possibilities for evolutionary change.
Diversification of life probably did not occur until after the advent of sexual reproduction, or until oxygen levels reached a critical threshold.

Eukaryotes appeared by Archean time as determined by molecular fossils or biochemical remains.

Fossils of the earliest eukaryotes are rare because they were microscopic single-celled organisms.

Eukaryotic cells can be differentiated from prokaryotic cells on the basis of size.
Eukaryotes tend to be much larger than prokaryotes (larger than 60 microns, as compared with less than 20 microns).

Larger cells begin to appear in the fossil record by 2.7 by to 2.2 by.

Eukaryotes began to diversity about 1.2 to 1.0 by ago.


Characteristics of acritarchs:

It is not known what sort of organism they were, but some resemble the cysts or resting stages of modern marine algae called dinoflagellates.

The First Metazoans (Multicellular Animals)

Metazoans are multicellular animals with various types of cells organized into tissues and organs.

Metazoans first appeared in the Neoproterozoic, about 630 my ago (0.63 by). Preserved as impressions of soft-bodied organisms in sandstones.

Examples of metazoan fossils in the Proterozoic:

Geologic time scale across the Precambrian-Cambrian boundary
Geologic time scale across the Precambrian-Cambrian boundary, showing the Ediacaran fauna and other faunas.

Types of Ediacara fossils

Oldest Ediacara-type fossils are from China.
Youngest Edicara-type fossils are Cambrian (510 my, Ireland).

Because the Ediacara creatures are not really similar to animals that are living today, this has led to the suggestion that they be placed in a separate taxonomic category or new phylum. The name proposed for this new category is Vendoza (named after the Vendian, or the latest part of the Neoproterozoic in Russia).

Ediacara fauna is an important record of the first evolutionary radiation of multicellular animals.
Some were probably ancestral to Paleozoic invertebrates.

Small Shelly Fauna: The Origin of Hard Parts

Small fossils with hard parts or shells appeared in the Neoproterozoic.

Cloudina, an organism with a small (few cm long), tubular shell of calcium carbonate, resembles structures built by a tube-dwelling annelid worm. It is the earliest known organism with a CaCO3 shell. Found in Namibia, Africa.

Cloudina, the earliest known calcium carbonate shell-bearing fossil.

Other latest Proterozoic and earliest Cambrian small fossils with shells include possible primitive molluscs, sponge spicules, tubular or cap-shaped shells, and tiny tusk-shaped fossils called hyoliths. Some early shelly material is made of calcium phosphate.

Precambrian Trace Fossils

Trails, burrows, and other trace fossils are found in late Neoproterozoic rocks around the world. In every locality they are found in rocks deposited after the Neoproterozoic Varangian glaciation.

Mostly simple, shallow burrows.

The trace fossils increase in diversity, complexity, and number in younger (Cambrian) rocks.

What stimulated the appearance of metazoans?

May be related to the accumulation of sufficient oxygen in the atmosphere to support an oxygen-based metabolism.

Ancestral metazoans may have lived in "oxygen oases" of marine plants.

Ediacaran life may have evolved gradually from earlier forms that did not leave a fossil record.

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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

November 8-10, 2005