One of the more common brachiopods to be found at Arkona is Devonochonetes sp. which is a small Strophomenid. It is very common at certain horizons of the Arkona and Widder formations.
This is a large example of the genera from the Arkona formation. Note the bumps along the hinge line which is a diagnostic feature and I may be able to use them to narrow down the species.
This is a smaller example that I believe comes from the Widder formation. This may also represent a different species from the example above judging from the radial line ornamentation on the surface of the shell and deeper curvature of the valves.
The size difference does not represent any known correlation between the formations, these specimens are a random sampling of the dozens of specimens I've come across. Below is a (literal) handful of specimens from both formations.