Recently, I have been collecting Silurian corals. I found this one which has several interesting aspects. One it shows four horn corals that have budding out of the main one and another coral that has budded out of its base. Second, the smaller horn coral at the base has holdfasts coming out of it.
It is this second point that merits my study today. I have been finding small horn corals with marks on their horn exterior surface and sometimes I find "spines" intact (see green arrows on first picture). At least that is what I assumed till I found this specimen. My thought was they were spines that kept snails from moving up the horn wall (corallum?) to the calice or horn rim. They reminded me of spines on crinoid calyx. I have found what appear to be snail boring holes or track lines on the corallum on other horn corals, so it made sense.
This fossil shows one of them intact that it is holdfast to anchor the smaller horn to the larger horn. So these protrusions were used as anchor lines to bond the corals together in this case.
These pictures where taken quickly under an incandescent light and I was not using a tripod. I will post more about this fossil later since it shows a good example of the budding reproduction process used by horn corals.
Reading up, the book calls the Silurian period, the "Age of Corals". From what I can tell the official coral of that period should be the Halysites chain coral. The Fossils of Ohio book identifies the following phylum has Silurian: Amplexus, Cyathophyllum, Holophragma, Rhegmaphyllum, and Zaphrenthis.