Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Cambrian
Eon Overview
The Cambrian Explosion
The Burgess Shale Fauna
"Age of Trilobites" and the Cambrian Fauna
Plate Tectonics at the Start of the Paleozoic
Ottoia prolifica
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Burgess Shale fossils
Cambrian algae
Archaeocyath reef
Cambrian rocks
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Foundational Concepts
Dating Methods
Earth Processes
Life Processes
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The Cambrian Explosion
The fossil record of Proterozoic life is sparse because it was largely soft bodied or microbial. The advent of the Paleozoic Era brought an incredible diversification of multicellular animals having many different body plans, including most of the major groups alive today, such as mollusks, trilobites, other arthropods, brachiopods, echinoderms, corals, sponges, and chordates (our ancestors). Most significantly, the first animals having hard outer skeletons evolved; as a result, many of these shells became fossilized. Most of the Early Cambrian animals having exoskeletons were small, measuring one to five millimeters, and are often called the "Small Shelly Fauna". Another unique feature of the Cambrian fauna is the sponge-like archaeocyaths that formed reefs during Early Cambrian time. This sudden appearance of many different kinds of animals having skeletons (shells) in the fossil record is called the Cambrian Explosion. The Cambrian Fauna is as different from Vendian life as it is from the Paleozoic Fauna that occurs above Cambrian-age rocks.

The diversity of Cambrian life contrasts with that of the late Proterozoic partly because the newly evolved exoskeletons allowed Cambrian animals to become fossilized more easily. The evolution of shells may have allowed for the evolution of a wider variety of body plans. Whatever the reason, Cambrian time saw an "explosion" of diversity in the evolution of life on Earth. Shells also provide protection, and the Cambrian records the first examples of predation in the fossil record. For many different reasons, the Cambrian Explosion is an important event in the history of life.

Overview | The Cambrian Explosion | The Burgess Shale Fauna
The "Age of Trilobites" and the Cambrian Fauna | Tectonics at the Start of the Paleozoic

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