Iron pyrite ammonite fossil

This is one of the fossils I found at the beach during my trip to Lyme Regis last weekend. I've cleaned it up a bit and will try to clean it some more, it is so sparkly and pretty!

The ammonite died, was covered in silt and over time the silt was compressed to rock and the dissolving shell was replaced by the mineral pyrite.

According to my sources, this chunk of rock is 200 MILLION YEARS OLD! I just can't get my head around numbers that large. Can you?