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Although technically all rugose corals were solitary animals, some grew in groups, such that their skeletons were touching. These groups of rugose corals formed mound-shaped fossils that can be difficult to differentiate from colonial or tabulate corals. In rugose mounds, each tube or corallite skeleton has its own skeletal wall, while corallums in tabulate colonies shared walls. Also, septae in rugose corals are longer and generally more complex then those in tabulate corals. In Kentucky, these types of corals are common in Silurian, Devonian, and Mississipian strata.