Ammonites an extinct group of marine invertebrate animals that (in a way) ruled the Earth’s seas for almost 250 million years. Ammonites thrived so much and they were so numerous that today they are among the best index fossils, and it is often possible to link the rock layer in which a particular species or genus is found to specific geological time periods.
Organisms are pyritized when they are in marine sediments saturated with iron sulfides. Pyrite is iron sulfide. As organic matter decays it releases sulfide which reacts with dissolved iron in the surrounding waters. Pyrite replaces carbonate shell material due to an undersaturation of carbonate in the surrounding waters.