Introduction to the Early Paleozoic

Pamela J. W. Gore
Georgia Perimeter College


  1. Precambrian-Cambrian Transition
  2. Cambrian

The PreCambrian-Cambrian Transition

To learn more about the Vendian, click here.

To learn about Vendian fossils, click here.

Placement of Precambrian-Cambrian boundary is problematic; usually below "shelly" faunas. Originally at first trilobites.

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The Cambrian Period

545 - 495 Ma

The Cambrian page from Berkeley.

Information on the Cambrian time scale and Cambrian stratigraphy from Berkeley.

Cambrian Life

Tectonic and climatic setting

See paleogeographic reconstruction in text

Spend a few minutes to locate the various continents and their names at this time.

As we proceed through the Paleozoic, we will be referring to these paleogeographic reconstructions frequently to see changes as time passes.

For the Cambrian, note that the continents are separate (not all assembled into Pangea until the end of the Paleozoic), and that they are primarily located in tropical latitudes.
Also note that there are NO CONTINENTS LOCATED AT THE POLES.
This will become important later in the Paleozoic because it influences climate.

What does this mean?
Basically, if there are no continents at the poles, there was no place for glaciers to grow. Global temperatures were probably warm, because tropical waters could not flow far before running into a continent, and being deflected N or S.

Also note the distribution of rock types, such as evaporites (E). Today, the desert latitudes are at about 30 degrees N and S of the equator. Note all of the shallow sea deposits. Most are tropical, suggesting the sort of warm, shallow sea conditions needed for the formation of limestones. Indeed, the Cambrian and Ordovician are times of widespread deposition of carbonates, particularly in the Appalachian region.

Check the Cambrian Paleogeographic map to see the distribution of sedimentary environments.

Cambrian Fossil Record

The Cambrian is best described as a time of evolutionary explosion. All marine invertebrate phyla appeared during this time, with the exception of the bryozoa. The dominant marine invertebrates with hardparts were the trilobites, brachiopods, and archaeocyathids. Tommotian (lowermost Cambrian)

archaeogastropods, brachiopods, sponges, archaeocyathids, hyolithids, worm-like taxa
First diverse biotas of animals with skeletons.

Between 570 - 520 my, nearly all coelomate phyla with skeletons appeared rapidly.

Origin of skeletons (shells)


  1. Support for muscles, etc.
  2. Protection against environment & other organisms, predators
  3. Aid in locomotion

Skeleton served as an adaptive breakthrough allowing preferential survival and niche exploitation.

Adaptive radiation occurred; rapid diversification of forms.

Possible reasons for the advent of skeletonization:
Increasing oxygen levels (Toewe, 1970)
Synthesis of collagen, a fibrous connective tissue in shells, cuticles, carapaces, etc. directly or indirectly requires collagen.

Oxygen priorities
Low oxygen levels: respiration & tissue synthesis
High oxygen levels: oxygen can be used for lower priority things - skeletons, shells, etc.
Oxygen reached critical threshold at this time, eliminating the need for priorities.

Evidence: modern low O2 environments have only small, soft-bodied forms.

Muscles require collagen, suggesting that early muscles were weak, BUT early evidence for metazoans shows burrows which would require good muscles.

Stanley (1976) disagrees. Says Cambrian was a time of the initial proliferation of organisms and life in general. Says skeletonization was a major evolutionary experiment, which succeeded, leading to rapid adaptive radiation and survival of the fittest.

The earliest organisms with soft bodies may have had respiration by diffusion across body wall.
When O2 content of atm. increased, it allowed them to develop respiratory systems, circulatory systems, etc. No longer needed to use O2 diffusion. Larger body size possible.

Sudden simultaneous appearance of skeletonized taxa.
Adaptive radiation. Follows Punctuated Equilibrium model.

Cambrian fossils:

Most Cambrian animals were herbivores that fed on algae.

A few worm-like animals and arthropods had specialized adaptations for feeding and may have been carnivores.

One of the largest animals to live during the Cambrian was Anomalocaris, about a half-meter long. It had a pair of segmented, spiny appendages near its mouth. Its mouth was encircled by a ring of 32 radiating plates, resembling a slice of pineapple, with inward-facing spines, functioning like teeth in a unique "jaw-like" apparatus. Its size and anatomy suggest that it was a predator. In addition, trilobites seem to have "bites" taken out of them that appear to have been inflicted by Anomalocaris. (Briggs, 1991, American Scientist).

The Cambrian was a time of "evolutionary experimentation" and adaptive radiation. Many body plans were tested, but only a few succeeded during natural selection (predation, competition, etc.).

Many groups of animals appeared, but few succeeded.

Trails: Climactichnites,
505 million years old, Late Cambrian,
New York

The Burgess Shale

530 my (Lower to Middle Cambrian)

Click here to go to Dr. Gore's Burgess Shale Page

In a few places in the world, there are examples of exceptional preservation of soft-bodied marine invertebrates. The Burgess Shale is the most spectacular of these. Its occurrence early in the Cambrian serves to highlight the great diversity and variety of multicellular life at this early point in its history.

Wide variety of body plans is shown in the BURGESS SHALE. Although hard parts had appeared, soft-bodied organisms were still present (as they are today).

Significance: exceptional, unique preservation of soft-bodied animals in the Cambrian.

Discovered by Charles Walcott, 1909, Canadian Rockies, British Columbia.

Good preservation indicates deposition in anoxic conditions. Many delicate details of soft part anatomy are preserved. (Legs and gills of trilobites, for ex.). Fossils found in turbidite beds in the Burgess Shale, which were swept off an adjacent, well- oxygenated, carbonate platform by turbidity currents, and killed and protected from decay in anoxic water.

More than 120 species. Includes previously unknown phyla. Arthropods, molluscs, echinoderms, coelenterates, onychophorans, priapulid worms, annelids, chordates, sponges.

References on the Burgess Shale:

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This page created by Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

Page created October 1995
Modified November 3, 1997
Modified February 3, 1999
Last modified July 17, 1999
Links updated August 15, 2009