4cm. A slice through the middle of a goniatite from Winterberg quarry near Bad Grund in Harz, Lower Saxony, Germany.
The stratigraphy is interesting. Although the micrite sediments in which they and other fauna were found are early carbiniferous, they were deposited in a tectonic fissure within the local devonian reef limestone, which made for a bit of riddle work when they were first discovered. Below is another one of them.