Rugose corals are a common fossil in Devonian Period rocks. They come in both solitary and colonial forms. Solitary rugosans house a single polyp within a single coralite made of CaCO3. Colonial rugose corals are forms in which many individual coralites were constructed directly adjacent to one another, inevitably sharing resources and ecospace. Solitary and colonial rugose corals were probably grew in soft sediment as long as they were able to keep their polyps above the sediment. In this way, the polyp, which was a filter feeder, would be able to sieve detritus from seawater without becoming clogged with suspended sediment.
Solitary rugosans were further specialized in their ability to grow as either upright or recumbent forms, creating the forms more commonly known as "horn corals". Some forms appear to have lived attached to hard substrates such as the shells of other marine invertebrates.