I was first introduced to Strophodonta demissa via fossils from the Silica Shale in Ohio. Subsequently I have found it in the Widder Formation and now from the Windom shale member of the Moscow formation. I found the below specimen along the shoreline of Lake Erie and it's the most inflated example I've found to date.
This is the first loose specimen that I've found from the Windom Shale but I have found other examples that are embedded in the rock or broke apart upon extraction. The Windom Shale is dated to the Givetian stage of the middle Devonian.