This is the heteromorph ammonite Aegocrioceras sp. which comes from Resse (near Hannover), Germany and Cretaceous (Hauterivian stage) rocks. Heteromorph ammonites are identified by their "loose" to non existent coils. Most Ammonites are tightly coiled with each subsequent coil overlapping the one prior. This Aegocrioceras may have started out with a tighter coil but as it grew the shell separated. I don't know if this gave it an evolutionary advantage or not but it looks cool.
Thanks to Nils for the specimen.