Trilobites roamed the oceans from 521 to 250 million years ago. They had a hard shell, with a head (Cephalon), a body (Thorax) made up of many segments, and a tail (Pygidium).
The start of their existence defines the start of the Atdabanian stage in the Early Cambrian period, and their disappearance defines the end of the Permian. They were a successful species for over 250 million years.
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