Agoniaties was a Devonian cephalopod that lived in the warm paleozoic seas. It's thought that, due to close association, these squid-like animals were predators of trilobites. Agoniatites from the Widder formation in SW Ontario are incredibly rare, the species being more common throughout New York State. This lineage of this ammonoid is scientifically important. It is considered to be an intermediate  species between the goniatites and the ammonites that would come to dominate the seas in subsequent eras.
 
Pyritization occurs due to the high concentration of the mineral in the host rock. Often, the living chambers are preserved in incredible detail.
 
Agoniatites vanuxemi
Widder fm, Hamilton gr
Arkona, Ontario, Canada
Middle Devonian