Protoleptostrophia perplana is a very thin shelled strophomenid variety of brachiopod. Often the only way one can find and preserve a specimen is when it's attached to a rock surface. Lucky for me this specimen was robust enough to weather out of the rock and be found on a scree field of rock from the Windom shale.
A smaller specimen that popped off a rock I was splitting. The fossil is so thin that light shines through it.
Both specimens were found in the Windom shale member of the Moscow formation (Devonian, Givetian stage).