Hyolithid
Hyolithes baconi
Middle Ordovician (Platteville)
Grand Detour Formation
Rock County WI USA
33mm hyolithid. Rare.

Hyolithes is a genus of hyolithid, a group of 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 did not recover as well as some other groups but remained widespread and survived into the Permian before becoming extinct.