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 During this time complex multicellular organisms became
well-established, both in the oceans and on land. The so-called
Cambrian Explosion, the first appearance of large numbers of
multicellular organism, had its roots in the preceding 100
million years or so of the PreCambrian. During the Cambrian true
tissues, bilateral body symmetry, digestive tracts,
segmentation, and shells and skeletons became widespread in
animal groups. As the Ordovician period progressed, ecosystems
became more complex as more and more niches were occupied by new
organisms. The Ordovician period ended with a major extinction
event, wiping out many existing organisms but setting the stage
for another adaptive radiation as the Silurian period began.
Learning Objectives
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Was the Cambrian explosion really an explosion?
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What geologic and climatic factors may have contributed
to the seemingly sudden appearance of complex
organisms in the Cambrian?
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What evolutionary features were important in the many
body plans that developed during the Cambrian?
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How were atmospheric oxygen levels important in the
early development of organisms and ecosystems?
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