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The Cambrian period

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The Cambrian period (from 541 to 485.4 million years ago) was a time of massive diversification of life in the ocean. It was thought to have begun as a result of changing ocean chemistry (due to erosion and minerals washing into the ocean) and a boom in oxygen levels due to growing populations of phytoplankton (see Chapters 7 and 8 for more about phytoplankton). During this period:

 Shells and exoskeletons with new body plans evolved.

 The first complex animals with mineralized remains formed.

 The evolution of flexible limbs became the first “arms.”

 The first real predators evolved, such as anomalocaris, a shrimplike creature that hunted worms and other soft bodied animals.

 Pikaia (see Figure 3-2) and haikouella grew a flexible rod of cartilage to swim, becoming the ancestors of the vertebrates (animals with backbones).

 Trilobites, one of the first arthropods (think spider or crab), appeared.

 More complex food webs began to develop.

This period ended with the Cambrian-Ordovician Extinction event, due to a possible crash in oxygen levels in the ocean.


Photo by Nobu Tamura. Licensed under CC BY-SA 2.0

FIGURE 3-2: Artist rendering of a Pikaia with a jellyfish.

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