Which extinct coral order is known as horn corals?

The extinct coral group known as horn corals is Rugosa.

Rugosans lived in marine environments from the Middle Ordovician to the end-Permian extinction. Solitary members often developed a tapered, horn-shaped skeleton, which is why they are popularly called horn corals. Their walls were characteristically wrinkled, giving rise to the name “rugose.”

Rugosa included both solitary and colonial forms. Some individuals, such as species of Caninia, Lophophyllidium, Neozaphrentis, and Streptelasma, resembled individual horns, while colonial rugosans could form substantial reef-building structures. Their fossil skeletons were made of calcite and preserve much of what scientists know about them.

Taxonomic rank can vary by classification: modern references commonly describe Rugosa as an extinct class, while older treatments may call it an order. That rank variation does not change the standard quiz answer. Rugosa is also distinct from Scleractinia, the order containing modern stony corals.