Horn corals, known as Rugosa or Tetracoralla, are an extinct order of coral that were abundant during the Middle Ordovician to Late Permian stages. They were known as horn corals because of a unique horn-shaped chamber with a wrinkled (rugose) wall.
These mostly solitary corals often reached lengths of nearly 40 inches. However, some species could form large colonies.
Rugose corals have a skeleton made up of calcite that is often fossilized. Like modern corals, rugose corals were invariably benthic, living on the sea floor or in a reef-structure. Although there is no proof, it is believed these Paleozoic corals possessed stinging cells to capture prey. They also had tentacles to help them catch prey. Technically these corals were carnivores, but prey-size was so small that they are often referred to as micro-carnivores.