This is an unusal coral that I found at Arkona a few weeks ago. It came from the Hungry Hollow member of the Widder formation which is middle Devonian in age. It appears to be several calices of the rugose coral Heliophyllum halli that show budding off one another. It makes a candelabra like shape and there is clear evidence that this piece once had more connected to it.
It's too bad the calice in the last picture is damaged slightly as otherwise the detail is perfect. Often I find these corals with sediment still within the calice and so fine details are hard to see.