Figure from Andrew H. Knoll and Sean B. Carroll , in Science V. 284, p. 2129, 1999.

The Cambrian Period represents a remarkable time in Earth's History during which life forms diversified and atmospheric composition approached modern conditions. 

The earliest Cambrian shelly fossils are known as the TOMMOTIAN FAUNA

Following the Tommotian fauna, larger and more complex hard shelled marine invertebrates developed and proliferated. Cambrian seas were dominated by trilobites (75-90% of Cambrian population) and brachiopods (5-15%), but also included archeocyathids (ancient reef builders), sponges, algae, bryozoans, echinoderms, etc.

Major Species include:

Paradoxides gracilis (trilobite), Hydrocephalus minor (trilobite), Peronopsis integra (trilobite) Stromatocystis pentangularis, & Acadocrinus nuntius

Figure source: Smithsonian Institution

The continued rupture of the Gondwana supercontinent provided wide continental shelves on four major continents. These shallow, sunlit platforms provided outstanding habitats for marine burrowers, swimmers and floaters. Suddenly, the seas were alive with invertebrate critters. This population explosion resulted in increasingly more complex predator/prey relationships, habitat development, defense mechanisms and food webs. Natural selection favored those organisms adept at adaptation.

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