Another common Epibont found on shells from the Silica Shale, and Devonian in general, is Aulopora. The 1975 book by Robert V. Kesling & Ruth B. Chilman: Strata and Megafossils of the Middle Devonian Silica Formation: Museum of Paleontology, Papers on Paleontology, No. 8, pg. 56, lists only one species that can be found within the Silica Shale: Aulopora microbuccinata. Aulopora is a species of coral and bears a similar growth pattern and overall look to the Bryozoan Hederella. The easiest way to tell them apart is that Aulopora is much larger.
Here is some growing on a Paraspirifer
Another colony on a Mucrospirifer
Another colony on a shell fragment
The corallites of Aulopora are thin walled and sometimes they are easily broken open by the erosion process.