Hexagonaria sp. - fossil rugose coral from the Devonian of Michigan, USA. (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA)

The “official state stone” of Michigan is the Petoskey Stone, the local name for rounded pebbles or cobbles of the fossil colonial rugose coral Hexagonaria percarinata. Petoskey Stones have been weathered & eroded from fossiliferous limestones of the Traverse Group (Middle Devonian). They are particularly common in the vicinity of Little Traverse Bay & the town of Petoskey (northwestern Lower Peninsula of Michigan). Beach & lake gravel in this area consist of many lithologies, including fossiliferous limestone clasts and worn fossil corals (favositids & colonial rugosans).

The city of Petoskey, Michigan gets its name from an old Ottawa Indian Chief, Ignatius Petosega. “Pet-o-sega” means “rays of the rising sun”. Petoskey Stones are so-named in reference to the dark-colored “eye” & radiating lines of individual Hexagonaria corallites.

From exhibit signage:

"Bob Hook Memorial Petoskey Stone"

This specimen represents a portion of a large colonial reef coral called Hexagonaria. The complete coral, in life, was at least two or three times the size you see here. Just like modern reef corals, each round cavity on the surface of the coral housed a soft coral polyp connected laterally to neighboring polyps. These corals flourished in shallow reef areas during Devonian (mid-Paleozoic) time.

This specimen was discovered and unearthed by Bob Hook along the Lake Michigan shoreline at Bay View, Michigan.

Classification: Animalia, Cnidaria, Anthozoa, Rugosa, Phillipsastraeidae

Stratigraphy: Traverse Group, Middle Devonian

Locality: shoreline of Lake Michigan, Bay View, Michigan, USA

See info. at: