Scientists estimate that the earth formed between 4.5 to 4.6 billion years ago.

The Pre-Cambrian era is one of the longest time periods on the prehistoric time line.  It covers 90% of the Earth's history. It lasted from the point of the Earth's creation to 570 million years ago ( the msu site says 600 million years ago). 

The next major era is the Paleozoic era. The Cambrian period was part of the Paleozoic era and produced an intense burst of evolution.  The first hard-shelled mollusks appeared during the Cambrian period, at roughly 545 million years ago. 

The important thing to consider when looking at these dates is that different scientists do not always agree on the prehistoric time line, so it is not unusual to have discrepancies on the dates.