A climate change question

Disclaimer: I am not a climatologist, geologist, astronomer, paleontologist or evolutionary biologist.

A climate change question… The atmosphere of the planet Earth has not always contained atmospheric concentrations of CO₂ equivalent to those of pre-industrial Earth.  Part of the reason Earth had such relatively low atmospheric CO₂ levels is due to the massive amounts of CO₂ sequestered in the form of oil and coal deposits below ground.  How did oil and coal sequester so much carbon underground?  My understanding, on a rather crude and elementary level, is that they are the organic remains of organisms which emerged from the Cambrian explosion most of which likely died out in the event which resulted in the extinction of dinosaurs.  If this is the case, how much carbon was in Earth’s atmosphere before, during and immediately after Cambrian explosion/extinction of dinosaurs?  What was the climate like at those times? Will the currently increasing CO₂ levels return the planet to similar climate conditions?

My mind wanders at times...