Any Horn coral, belonging to the order Rugose is called Rugose coral. The Rugose possesses a wrinkled exterior portion, and it derived its name from the Latin word which means wrinkled. It is often called as a Horn coral due to the horn like shape in which the corals are found. The horn corals reside in a cup-shaped calyx. It is either colonial or solitary. The fossils of Rugose belonged to the Ordovician era, which means, around 488 million years ago. The species existed till the end of the Permian era, which is just 251 million years before. The fossils of some of the Rugose coral forms are used as a guide or index to study the geological time and to calculate the length of geological day and year. Some are also used to study the relation between the units of separated rocks. The Rugose skeleton is a formation of calcite, which is often acquired as a fossil.
The Rugose corals are seen in a cup-shaped calyx. This calyx possesses grooves and ridges in an aligned form which are referred to as septa. It possesses tabulae or the horizontal plates, and sometimes it has dissepiments or the curved plates, which are connected to tabulae and septa. The corallite skeleton is divided by the horizontal plates. The Rugose corals always show bilateral symmetry and this symmetry can be recognized by the development of the septa, which is seen in a transverse section. In the Rugose corals, both minor and major septa can be found. In the beginning, there are only 6 septa in a major form and gradually, the small form of septa develops in the remaining 4 spaces. This unique fourfold arrangement of the septa, gives another name of Tertra corals to this genus. The solitary growing Rugose corals need extra support, hence it always possessed a columella which is an axial bar or rod that supports the septa.
The Rugose coral was generally seen to develop a strong skeleton. Converging trends were seen in different lineages to develop similar anatomy. Many lineages developed Columella and Carinae to support and strengthen the septa. During the Ordovician period, the simplest forms of Coralite group were found, which possessed tabulae, simple walls and septa. The polyp was found on the top portion of the tabulae. Gradually, new forms developed with a thickened calcite layer near the corallites' periphery, which was surely there to give support to the organism. Till the early Silurian period, the rugose corals were found in small forms and in a solitary condition, but just after the first phase of the Silurian period, they started to appear in a colonized form. In the Devonian era, few forms from the Silurian period survived. A new radiation of adaptation occurred in the Rugose corals of the Middle Devonian era. By the end of the Devonian period, a mass extinction of species took place which severely affected the coral fauna. During the Carboniferous period, the skeleton's microstructure developed into a quite complex form. It is believed that the Rugose coral of the Carboniferous era was the most advanced form till then. In the Permian period, the last radiation of these corals took place, which led to many varieties of species. But by the end of the Permian period, a huge extinction event referred as the Permian-Triassic event took place and this terminated the entire class. The fossils of these corals are commonly found in the Beil Limestone in the areas near Sedan, Kansas.