The Cambrian period is a part of the Lower Palaeozoic (see below):

PALAEOZOIC
UPPER
Permian
45,000,000 yrs
280 x 10E6
Carboniferous
65,000,000 yrs
345 x 10E6
Devonian
50,000,000yrs
395 x 10E6
 
LOWER
Silurian
35,000,000 yrs
430 x 10E6
Ordovician
70,000,000 yrs
500 x 10E6
 
Cambrian
70,000,000 yrs
570 x 10E6

The fourth column shows how long the period lasted, the fifth column shows how many years from the present it was current. The term 10E6 means one million for those of you who are not scientifically minded.

It is not until well up into the Cambrian period that fossil remains are found. There are schools of thought that say the sea water of the Pre-Cambrian did not contain enough dissolved salts to allow for secretions that make up the hard parts of animals.

So..Trilobites appeared in the Cambrian, reached their height in the Ordovician, after which they began a slow decline and the last known trilobites were found in rocks of Permian age on the Isle of Timor and in the Salt Range of the Himalayas.

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