Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Cambrian
Contents
Eon Overview
The Cambrian Explosion
The Burgess Shale Fauna
"Age of Trilobites" and the Cambrian Fauna
Plate Tectonics at the Start of the Paleozoic
Evidence
Ottoia prolifica
learn more
sample
Mollusks
Burgess Shale fossils
Hallucigenia
Trilobites
Cambrian algae
Archaeocyath reef
Wiwaxia
Cambrian rocks
references and links
Foundational Concepts
Dating Methods
Earth Processes
Life Processes
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The Burgess Shale Fauna
Although the Cambrian Explosion is largely associated with animals having hard shells, the soft-bodied biota also diversified during this period. The Middle Cambrian Burgess Shale fauna, discovered by Charles Walcott in 1909, in British Columbia, Canada, provides a rare glimpse of Cambrian soft-bodied animals. This fauna is especially important because the fossil record is biased toward organisms having hard parts. Another famous soft-bodied occurrence is the Chengjiang Fauna, of Early Cambrian age, in Yunnan Province, China. Although well-preserved soft-bodied fossils have been found throughout the fossil record, they are rare occurrences and are preserved only under very unusual environmental conditions.

The Burgess Shale was deposited in the ocean near an underwater algal reef shelf. Occasional undersea landslides buried the animals living there, and the fine mud prevented decay so that soft parts were preserved. As a result, this fossil fauna records a host of extinct soft-bodied organisms, many of which have no living counterparts. Additionally, the Burgess Shale contains many shelled animals, some of which preserve parts not normally seen, such as the legs and antennae of trilobites and the setae of brachiopods; the exceptional preservation allows paleontologists to understand the structure and biology of the animals better. Some of the soft body plans of Cambrian animals did not survive beyond that period. To our eyes, some of these body plans look bizarre and it has taken a long time for paleontologists to understand them. For example, the first reconstructions of the onycophoran Hallucigenia were upside-down. The sclerite-bearing Wiwaxia, the arthropod-like predator Anomalocaris, and the five-eyed Opabinia are all strikingly different from modern animals.




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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