Ammonites were amongst the most successful marine animals during the Mesozoic. They evolved to fill a large variety of ecological niches across a wide spectrum of open-ocean and marine shelf environments. Despite the fact that hundreds of thousands have been collected in the last 200 years and are available for study in museum collections, the biology of ammonites—how they lived—is very difficult to understand. They lived such exotic and strange lives comparable with no group alive today, and seemed to break all the rules of Darwinian biology. Their importance in stratigraphy is due to their rapid evolution, which gave rise to a more refined zonal scheme than is possible with any other group of fossil, making the need to understand ammonite biology of more than academic interest.