Hyolithes tenustriatus
Hyolithid
Cambrian
Krekling, Norway

Huge 78mm hyolith on 109mm matrix. Only one we have ever seen.

Hyolithids are shelled invertebrates of uncertain classification though the latest research places them perhaps related to brachiopods.  They reached their greatest abundance and diversity during the Cambrian but suffered greatly during the mass extinction at the end of that period.  The group went into decline but still lived in many parts of the world, managing to survive into the Permian before becoming extinct.